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American Mineralogist: Journal of Earth and Planetary Science:

All our papers are special, but each month, the American Mineralogist editors pick a few to be "Noted Papers". We hope this information is enjoyable and useful.

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Editors Selections, June 2017

Highlights and Breakthroughs

Anthropogenic Minerals

On page 925 of this issue, Peter Heaney provides a provocative overview of the immensely popular article by Hazen et al. (2017; this volume) regarding minerals that have an anthropogenic parentage. As Heaney notes, textbooks have largely given up on the notion that minerals form only by inorganic processes; he then asks whether we are “prepared to take the next step”: to accept synthetic crystals as minerals (as already proposed by Zalasiewicz et al. 2014). Hazen et al. do not require such in their definition of an Anthropocene epoch they attempt to play by IMA rules. Some of the most intriguing arguments appear to begin with how hypothetical visitors to, or evolved species on, a late Earth might classify crystalline structures. What is technology at present is archeology to a future age. What is uniquely anthropogenic now, may be a primitive step along a biogenic continuum to some future species. As our language evolves over centuries or even decades, it is sure to vary across species and millennia.

 

Diamondiferous Clues to a Non-chondritic Lower Mantle

On page 927 of this issue, William Bassett reviews a new work by Kaminsky and Lin (2017; this volume) who speculate on the composition of Earth’s lower mantle, based on the compositions of minerals included in diamonds and their synthetic analogs as produced in high pressure experiments. In the new work, Kaminsky and Lin (2017) examine the co-existing compositions of ferropericlase and bridgmanite (the latter of which is quenched to an orthopyroxene structure, but is presumed to preserve the composition of the high pressure, proto-mineral phase). They find that in some inclusions, the total amounts of Fe in both phases are high relative to what might be expected had they equilibrated from a pyrolite bulk mantle composition. And yet another set of inclusions contain lower-than-expected Ni. These observations lead Kaminsky and Lin to infer that the lowermost mantle may contain an Fe-rich zone that might even be saturated in a metallic Fe phase that may retain Ni. This suggestion carries with it the implication that mantle plumes from the lowermost mantle should yield volcanic rocks that are enriched in Fe, but depleted in Ni and other siderophile elements.  

 

Defining Chemical Bonds

On page 928 of this issue I. David Brown discusses a new paper by Bickmore et al. (2017; this volume), which attempts to connect bond energy to electrostatic characteristics, in this case electronegativity. The work is inspired by the observation that bond energies vary greatly across bond types. Perhaps the most intriguing implication is one left unsaid by the authors: they provide a possible fifth explanation for the rarity of many minerals, as enumerated by Hazen and Ausubel (2016; Am Min). Those authors suggested that rarity is typical for the more than 5000 minerals yet named, due to restricted P-T-X stability, planetary constraints, ephemeral stable conditions and collection bias. Bickmore et al. (2017) provide, if not an additional cause for rarity, then at least a possible rationale for ephemeral P-T-X stability, by postulating that peroxides are rare but persuflides are common. Peroxides are built upon O-O bonds, which have intrinsically less energy per bond valence than more ionic M-O bonds.

 

Not So Deep Diamond Inclusions?

On page 929 of this issue, Andrew Thomson reflects on some new and important observations by Uenver-Thiele et al. (2017) regarding the putative depths of mantle-derived diamonds. Some diamonds are thought to derive from depths of at least >800 km, on the basis of their containing magnesiowüsite (Mg,FeO), a phase that is not expected to be stable under subsolidus conditions in a pyrolite bulk composition (but is stable to atmospheric pressure in other bulk compositions). Even greater depths are suggested where some magnesioferrite phases occur as exsolution lamellae within Mg,FeO—but such inferences have not verified by experiments at ambient pressure. Uenver-Thiele et al. (2017) conduct the needed experiments and ironically find that such exsolution relationships are likely to occur only at P<10 GPa. Their new work does not preclude entrapment of inclusions at greater depths, but indicates that the exsolution textures in question represent an upper mantle phenomenon.

 

The Genesis of Arc Magmas

On page 931 of this issue, Christy Till reviews the conditions under which arc magmas are generated in the mantle wedge. She finds that while published P-T estimates of arc magma genesis vary greatly, the largest source of variation in these estimates is differences in their calculation methodology.  Once calculated with a consistent approach, wet primitive arc magmas reveal origins via water-saturated partial melting at 20-35 kbar, with subsequent equilibration (prior to transport to the crust) over relatively narrow P and T intervals of 8-19 kbar and 1075-1300 °C.  Anhydrous arc magmas reveal similar depths of mantle equilibration en route to the surface but at higher temperatures (1290-1450 °C), facilitated by channelized rather than porous flow. Till finds that these processes are sufficient to explain a rather wide range of arc magmas, from boninites to high Mg andesite to hydrous calc-alkaline basalt, leaving a lherzolite residue for the latter and a harzburgite residue for the former two. More importantly, though, Till is able to show how these P-T conditions can be mapped into different and distinct parts of the mantle wedge, which means that various volcanic composition suites may be used to describe the evolution of an ancient or evolving arc system. 

 

Little Water is Transported Into the Mantle By Subuducting Basalt

On page 975 of this issue, Schmädicke and Gose measure water contents in both garnet and omphacite in coesite-bearing eclogite. A key inference from their work is that molecular water in these phases is secondary, related to fluids that come into contact with the system following peak metamorphism. (The opposing possibility is that molecular water represents structural water that is exsolved upon decompression). In addition, molecular water may even secondarily enhance the content of structural water. In this case, only structural water of samples devoid of molecular water is useful for estimating water budgets in a downgoing basaltic slab, and these authors thus infer that such contents are quite low, on the order of 300-650 ppm, with most pre- and syn-metamorphic water being released to hanging wall rocks (e.g., the mantle wedge or exhuming high-pressure slices).

 

Remaking Zircon

On page 1066 of this issue Pidgeon et al. examine the recrystallization of radiation-damaged zircon. The recrystallization process first involves the formation of tetragonal zirconia, which then reacts with silica to recreate zircon. Their experiments indicate, however, that such recrystallization is highly unlikely in natural systems, and Pb and He loss would thus be permanent. They further note that recrystallization would be no more likely in synthetic zircons that are used to sequester radioactive waste, in which case radionuclides would be not be protected against reactions with fluids.

 

Letters

Animal, Vegetable, Mineral

On page 1129 of this issue, Hummer et al. provide the structure of one of the very few known organic minerals, in this case, a naturally crystalline geoporphyrin (porphyrins being a type of organic compound) called abselonite. This and related compounds are believed to be due to the breakdown under diagenesis of chlorophyll a, and the transformation to abselonite involves a highly specific change of cations from Mg to Ni(II).

 

Editors Selections, April, 2017

Review

How to Dissolve Kidney Stones

On page 701 of this issue, Hill et al. present a new computer model, based on current thermodynamic measurements, that can be used to predict the solubility of Ca phosphates in body fluids. As noted by the authors, the human body must in places remain supersaturated in hydroxyapatite for healthy bones and teeth but that oversaturation in the kidneys can lead to the development of unwanted stones. This new model attempts to predict mineral saturation under some of the various complex fluids of the human body, and so better inform the conditions that lead to kidney stone development, and thus better inform preventative measures.

 

Articles

Found: Magmatic graphite

On page 728 of this issue, Perez-Soba et al. present a case for what they argue is the first occurrence of graphite (in a granite host) that is of magmatic origin. Their study focuses peraluminous granites of the Variscan Belt, and three different types of magmatic apatite contained in these. Because peraluminous compositions have a high solubility for apatite, these minerals tend to record late stage processes. They find that some fluorapatite grains of apparent magmatic origin contain needle-like inclusions of graphite. They interpret the graphite inclusions as forming at a very late stage of crystallization, where the fluorapatite is in equilibrium with a fluid phase and two immiscible (perphosphorous and peraluminous composition) magmatic liquids and they suggest that a C-rich system may induce liquid immiscibility. These intriguing results point to the usefulness of apatite as a means to investigate the P-T conditions of very late-stage magmatic processes….

 

An Apatite Barometer

On page 743 of this issue, Ashley et al. calibrate an equation of state for complex apatite solid solutions. They find that their equation of state may provide a powerful tool to obtain P estimates from apatite included in garnet. They find that garnet is an ideal host in that this isotropic mineral also has a very high bulk modulus, and so more ably preserves the stress state of included minerals. Their tests further indicate that the pressures recorded by apatite are not highly sensitive to T. Apatite inclusions in garnet may this provide an accurate barometer in whatever systems they may be found, and may be especially useful when P-sensitive exchange equilibria are absent.

 

Special Collection: Olivine

A Precise Ni-in-Olivine Thermometer

On page 750 of this issue, Pu et al. calibrate a new Ol-liquid thermometer, based on the partitioning of Ni between Ol and co-existing silicate melt. The thermometer is calibrated with some several limitations: it uses experimental data collected at 1 bar only, of liquids saturated only with Olivine, containing ≤0.1 wt% NiO. But the resulting experimental calibration data set still spans a wide range of T (1170-1650 °C) and liquid compositions (37-66 wt% SiO2), and the resulting model ably predicts T from experiments performed at 10 kbar, and so may have an advantage over Mg-exchange based thermometers in requiring no knowledge of P (or rather to allow P to be estimated, at least at P<10 kbar, by determining the P at which the Ni and Mg exchange thermometers yield identical temperatures). Finally, their study also hints at the possibility that their Ni exchange thermometer may also be only slightly dependent on magmatic water (H2O, more appropriate than water) contents, thus offering the promise of a highly versatile thermometer that requires minimal input parameters for application.

 

 

Special Collection: Dynamics of Magmatic Processes

Water from Recharge Magmas May Help Expel Felsic Melt from Mush
On page 766 of this issue, Pistone et al. conduct rather clever experiments to examine the role of recharge magmas in aiding the expulsion of melt from overlying felsic mush systems. Their experiments, which place aphyric, hydrous andesite below a dacite mush, show a significant transfer of water from the andesite to the dacite, and in increase in the SiO2 contents of dacitic mush glasses as the addition of water apparently drives the dissolution of quartz in the mush. The authors suggest that mush rejuvenation occurs in two stages, the first dominated by heat transfer and the second by water transfer, from the mafic recharge magma to the felsic mush. Both these processes lead to an increase in melt fraction in the dacitic mush, and a decrease in melt density and viscosity, which are expected to aid felsic melt extraction.

 

Determining Impact Histories on Ancient & Weathered Planetary Surfaces

On page 813 of this issue Montalvo et al. survey some 11,000 zircon grains recovered from sedimentary deposits that may have the Verdefort impact site as a possible provenance. From that survey, the authors recovered three zircon grains that show tell-tale signs of shock-induced formation ({112} twins), with age dates of ca. 3 Ga that are consistent with the bedrock ages of the impact site. This work reveals the possibility for determining impact locations (and apparently maximum ages for impact) by examining the detrital zircon record.

 

 

A Non-Pyrolitic, Fe-rich Lower Mantle?

On page 824 of this issue, Kaminsky and Lin compare and find discrepancies between ferropericlase (fPer) and nominal bridgmanite (Bridg) (transformed to enstatite, so not true Bridg) compositions that occur as inclusions in diamonds, and as obtained from experiments. Some experiments are performed on non-Al bearing bulk compositions, and this work implicitly shows why experiments performed on natural compositions are so crucial. In any case, the authors find that some natural fPer have both high Fe and low Ni, from which the authors infer equilibration with a Fe-Ni alloy, a phase that presumably only inhabits the deepest mantle. They also find that certain nominal Bridg grains exhibit high Al, which is also inferred to be a lowermost mantle signature, on the expectation that Al-in-Bridg increases with increased P. The highest Al-in-Bridg crystals do not coexist with the lowest Ni-in-fPer grains, so the precise interpretation is unclear, but these compositions may still indicate important heterogeneity in the lower mantle. And while pyrolite is a highly flexible term (intended only to indicate rough proportions of basalt and peridotite), this study may still lead to the identification of distinct mantle domains—which if real, should connect to ocean island basalt compositions, or at least those exhibiting high mantle potential temperatures.

 

Wavellite as a Near-Surface Sink for Fluorine

On page 909 of this issue, Kampf et al. describe the new mineral, fluorwavellite, and its solid solution behavior relative to the non-fluorinated wavellite. In surveying wavellite from various localities, the authors find complete solid solution between the two end-members, but no connection between F content and mode of occurrence (e.g., F contents are not different for minerals obtained from pegmatites, ore deposits, or hydrothermally altered systems). The authors thus infer that F contents are controlled by the activities of Al, P, and F, and perhaps also by pH, since wavellite-fluorwavellite are highly soluble only under the uncommon conditions of pH>9. At lower pH, the authors suggest that wavellite-fluorwavellite is an important near-surface sink for F.

 

Editors Selections, March 2017

Special Collection: Olivine

Ni-in-Ol at Hawaii is Unrelated to Mantle Source

On page 507 of this issue Lynn et all examine NiO contents of high forsterite (Fo88) olivine crystals from various Hawaiian lavas (from Koolau, Kilauea, Loihi, Mauna Kea, and Mauna Loa). They find that over a 2.5X variation at individual volcanoes, Ni-in-Ol contents have no relationship to either host whole rock compositions, and that inter-island whole rock Ni contents strongly overlap and are not correlated to Ni contents in Ol. These authors show that variations in Ni-in-Ol at high Fo can be explained by very minor variations in the Ni contents of parental magmas (0.09-0.11 wt% NiO), and that observed ranges in Ni-in-Ol can be explained by a combination of fractional crystallization and magma mixing, and that to the extent that Ol may record mantle mineralogy, large, oriented crystals are needed to correct for intra-crystalline diffusion.

Special Collection: Apatite: A common mineral, uncommonly versatile

A Potential Oxybarometer Based on S in Apatite

On page 548 of this issue, Konecke et al. examine the oxidation states of S in natural apatite and experimentally equilibrated apatite + liquid pairs. Their work reveals perhaps the first report of a mineral that can incorporate three oxidation states of S (S2-, S4+, and S6+), which apply to experimental systems at oxygen fugacities ranging from FMQ to FMQ+3. These results show the potential for calibrating S6+/S4+ and or S4+/S2- ratios as oxybarometers for apatite saturated systems, although such a function is not calibrated, it probably can be from the data presented. The authors also note that the partitioning of various oxidation states of S may, in part, explain the non-Henrian behavior S portioning in apatite.

Outlooks in Earth and Planetary Minerals

Minerals Help to Define the Anthropocene

Most Am Min readers will know that some 400 news organizations have already covered the work of Hazen et al. (2017), which appears on page 595 of this issue. Hazen et al. show that 200 new minerals that occur exclusively or primarily because of human activities, may leave a decipherable stratigraphic signature, visible to future mineralogists. Some durable crystalline materials might not be classified as minerals today, as they are intentional products of engineering or social activities (Portland cement, semiconductors, various abrasives, etc.) rather than products of nature. Other substances at the boundary of natural and human activities occur because of human influences on near-surface conditions. Future mineralogists may be unable to decipher whether such substances were intentionally manufactured. Nevertheless, they are so ubiquitously distributed across Earth's surface that they will leave a robust mineral-like signature of what may be termed the Anthropocene.

Articles

Magnesioferrite Delimits Diamond P-T conditions

On page 632 of this issue Uenver-Thiele et al. present new experimental studies of the stability field and breakdown reactions of magnesioferrite (MgFe2O4). They find that above 8 GPa and 1000 degrees C it breaks down into Fe2O3 and MgO, and at yet higher temperatures, the reaction involves an unquenchable phase with an approximate stoichiometry of Mg5Fe2O8 or Mg4Fe2O7. Their results may shed light on magnesioferrite inclusions in diamond (often included in (Mg,Fe)O). Either such inclusions formed within the upper mantle, or if a lower mantle genesis is inferred, say from other phases, then the magnesioferrite must have formed from some other precursor phase.

Letters

Melanophlogite: A New Target For Investigating Life on Mars

On page 686 of this issue, Lazzeri et al. study the isotopic ratios of N and C in the silica clathrate mineral, melanophlogite (from Italy and California). This mineral, which apparently likes Mediterranean climates, also appears to occur in organic-rich settings and may thus preserve records of the biogeochemical cycling of N and C at or near Earth's surface. In their samples, the authors find N isotope ratios influenced by isotopic exchange with organic matter, and so suggest that the N was previously biologically processed. They further suggest that, if melanophlogite or similar microporous phases were to be found on Mars, their N isotopic compositions could be valuable as tests for modern or ancient biological processes.

Alien Alloys

On page 690 of this issue, Ma et al. report three new minerals from the Khatyrka CV3 carbonaceous chondrite. These minerals are alloys in the system Al-Fe-Cu: hollisterite, kryachkoite, and stolperite. This meteorite continues to reveal new surprises, being the host of icosahedrite and decagonite, the first two quasicrystalline minerals.

 

Editors Selections, February, 2017

Highlights & Breakthroughs

A Rover With a View (to the Early Martian Surface)

On page 233 of this issue Joshua Bandfield provides a perspective on the new mineralogical study by Ruff and Hamilton et al. p. 235 of this volume) on the pre-3.7 Ga Columbian Hills. Their study compares the mineralogy of two ancient surfaces, one more greatly affected by hydrous weathering than the other. They find that Martian weathering, at least in this one region, involved little cation removal, and so little in the way of quarts or phyollosilicates, or the production of amorphous Si phases such as opal. As Bandifeld notes, the attention drawn to Martian phyllosilicates and various hydrous amorphous Si phases is out of proportion to their scarcity on the Martian surface. Instead, the style of weather implied by the mineralogic study of Ruff and Hamilton, is one quite foreign to Earth where even in its coldest, driest parts, phyllosilicates and amorphous Si phases are common. On Mars, then, despite the evidence for aqueous erosion, the dominant weathering theme may be one of acid fog, however apparently contradictory these erosion and weathering themes may be.

 

Special Collection: Geology and Geobiology of Lassen National Park

Mantle or Crust Explains high d18O?

On page 252 of this issue, Underwood and Clynne present new d18O data on Ol, CPx, and Plag grains derived from mafic lavas of the Lassen Volcanic Center of the southern Cascades. Their investigation plays into the larger question of whether elevated d18O in whole rocks indicates contamination of such by a crustal source, or partial melting of a heterogeneous mantle. They find that d18O is uncorrelated to radiogenic isotopic ratios (Sr, Nd, and Hf) as well as trace element proxies for subducted fluid inputs (Sr/P), but isotopic ratios allow that the range of observed d18O may be mantle derived. Clearly the next step is to compare d18O in olivine to forsterite contents, which should provide a near-decisive test of whether high d18O ratios reflect high-T mantle or low-T crustal processes.

 

Special Collection: Dynamics of Magmatic Processes

Phenocrysts and Groundmass Age Dates Delimit Magma Storage Times

On page 262 of this issue Casalini et al. present new isotopic and additional geochemical data on the Ischia Volcano of southern Italy. Most fascinating is their Figure 10, where they find different 87Sr/86Sr ratios for groundmass and phenocryst phases, which based on analyzed Rb/Sr ratios permit to calculate crystallization intervals, which is obtained when groundmass and phenocrysts are nominally in isotopic equilibrium. The mineral residence times are then inferred from K-Ar-derived eruption ages. Their method yields time scales of magma storage and crystallization that range from tens to hundreds of thousands of years, with the most evolved magmas being stored in isolated pockets at cool temperatures, of ca. 750 C. These time scale estimates at Ischia provide the first radiogenic isotope probe of the novel approach of Cashman and Giordano (2014).

 

Special Collection: Water in Nominally Anhydrous Minerals

How OH- is incorporated into Olivine

On page 302 of this issue, Blanchard et al. re-examine the nature of water solubility in olivine (as OH-). Through a range of spectroscopic and computational methods the authors show that OH- solubility is controlled less by total Fe contents than by the total amounts of tri- and tetra-valent cations. Their work indicates that prior studies of OH- solubility can be re-interpreted so as to indicate that defect sites populated by OH- are associated with Ti4+ and Fe3+ in particular, and that H is dissolved mostly as a hydrogarnet species, where 4H occupy a tetrahedral vacancy and only more rarely as 2H occupying an octahedral vacancy.

 

Articles

CaSiO3-Perovskite: The Engine that Powers Mantle Convection

On page 321 of this issue Perry et al. use ab initio calculations to investigate the solubility of Th, and U into CaSiO3-perovskite with and without Al. They find that enthalpies of solution favor the dissolution of Th4+, and especially U4+, as coupled substitutions with Al3+. Their work suggests that subducting slabs may provide a key mechanism for transporting heat producing elements into the deep mantle. Subducting slabs host Ca-Al rich minerals that promote the storage and transport of U and Th leading to the formation of Al-bearing CaSiO3-perovskite as slabs penetrate the lower mantle. The authors also suggest that their findings may help the recipes for synroc compositions so as to optimize their chemistry for U and Th dissolution into perovskite-like structures.

 

Near-surface kerogen in deep-seated diamonds?

On page 391 of this issue, Childress and Jacobsen investigate the stability of kerogen at simultaneously high P and moderate T conditions. They find that increases in pressure allow an increase in the thermal stability of kerogen such that the coolest portions of subducted slabs can provide a pathway for carbon into the mantle. Their work provides a possible mechanism to explain recent findings of apparently organic carbon in some deep-seated diamonds.

 

Igneous or metamorphic hornblende?

On page 436 of this issue, Challener and Glazner, in a stunningly colorful contribution (pictorially, not in prose), examine hornblende compositions from the Half Dome Granodiorite of Yosemite National Park, California. Some of the crystals they examine are quite large, ranging to 2 cm in length, and are euhedral and un-altered in appearance. And yet, these authors suggest that these crystals record mostly a metamorphic history, rather than igneous one. In their interpretation, igneous crystals of the same size and shape were metamorphosed to greenschist facies conditions. Their evidence includes the bulk Hbl compositions, which are equivalent to hornblende plus a small proportion of biotite. They suggest that the original, igneous Hbl reacted with biotite inclusions, and perhaps quartz (rare as an inclusion, presumably due to reaction), to produce magnetite, alkali feldspars and clinozoisite, which are common as inclusions. Their interpretation is supported by exsoluiton textures indicative of low temperature equilibration.

 

P-T Conditions at the White Sands Nuclear Detonation Site

On page 445 of this issue, Lussier et al. examine the glassy arkosic sandstone called Trinitite, a synthetic product of 16 July 1945 nuclear test at the White Sands Proving Grounds in New Mexico. As noted by the authors, Trinitite has mostly been studied as a curiosity, but renewed interest involves its use as a test case for future forensic studies that may be needed to uncover bomb design, fuel material, etc., and trace these to a user of such. They find that quartz and zircon are both reliable recorders of blast P-T conditions and that nanoscale analyses of zircon grain (featuring remarkably dendritic rims — well worth a look at their Fig. 1) record the changing P-T-t conditions as a blast evolves. In this particular case, the authors infer T of >1500 C and P <10 GPa, the latter being quite below that of a meteorite impact.

 

 

Editors Selections, January, 2017

Highlights & Breakthroughs

Periodic Arcs

On page 1 of this issue, P.G. DeCelles reviews the new work of Kirsch et al., published on page 2133 in Am Min in 2016. DeCelles notes that while the rate at which magmas are produced in arcs is stubbornly uncorrelated with orthogonal convergence rates, Kirsch et al. provide an opening for a detectable tectonic control on magmatic addition rates at arcs. This results, in part, through the observations by Kirsch et al. of synchronicity of some (but not all) magmatic flare-ups throughout the Cordillera. As DeCelles notes, though, the mappable and datable effects of plate convergence are subject to many more factors than plate convergence rate. Implied is that convergence rates may provide an ultimate cause of magmatism and upper plate deformation, but are separated from intervening, and highly localized controls and conditions such that causative forces are well hiddenand may remain so absent detailed and comprehensive field and petrologic studies.

Invited Centennial Articles

Predicting Mining Accidents

On page 3 of this issue, Ulrich Bismayer provides an overview of a paper that we highlighted from last months issue: Jiang et al.s acoustic emission experiments on sandstone and coal lithologies. The larger sample size in these experiments allowed the investigators to detect a temporal transition. Early acoustic emissions are randomly scattered about the experimental volume and appear to be random with respect to both time and space. But later acoustic events cluster along what will prove to be collapse planes. And as noted earlier, the energy of these two event systems follow a power law, with a distinct exponent for the random and spatially correlated cases. These provide a means to predict failure events.

Invited Centennial Articles

Why Arc Lavas Contain High LILE

On page 5 of this issue, Hans Keppler examines melt inclusion compositions from primitive arc lavas. His review reveals that fluid mobile elements in arc lavas are, perhaps unexpectedly, controlled by fluids. These elements, which include the large ion lithophile elements (LILE), light rare earth elements (LREE), and U, are correlated with Cl when both Cl and the element of interest are normalized to H2O. This correlation with Cl indicates that the classic enrichment of arc lavas in LILE, LREE, and U are not controlled by subducted sediment inputs or partial melting of subducted crust, since Cl does not affect mineral/melt partitioning. Keppler concludes that (Cl-bearing) fluids are the primary carriers of LILE, LREE, and U into arc magmatic systems, and that Ce/H2O ratios are a proxy for fluid inputs, not slab T.

Articles

Carbon in the Lunar Core

On page 92 of this issue, Steenstra et al. suggest that Carbon is the major light-alloying element in the lunar core. Like Earths core, there is a recognition that the Moons metallic portion has sufficiently low density to require an admixture of elements other than Fe and Ni. And like Earth, sulfur has been a leading candidate of a light-alloying element as it readily dissolves into metallic liquids. These authors use existing estimates of the bulk silicate Moon, and recent experimental work that describes metal/silicate partition, to show that C may compete successfully with S during lunar core formation. These authors contend that C is sufficiently siderophile to allow up to 4.8 wt% C in the lunar core. This work further indicates a close similarity between the bulk silicate portions of Earth and its Moon, and a lack of devolatilization during the Moons formation.

The Ecology of Cobalt

On page 108 of this issue Hazen et al. examine the temporal and spatial distributions of Co-bearing minerals (66 distinct species; >3000 species-locality pairs). Their work indicates that Co-bearing minerals follow a Large Number of Rare Events (LNRE) distribution, which is plotted as a ranking of minerals according to the numbers of localities at which they are found. At the top of such a list (if applied to all minerals), would be quartz, which is found at 45,000 localities; 22% of all minerals are found only at one locality. Hazen et al have previously shown that an LNRE distribution describes minerals as a whole, and minerals characterized by elements that are concentrated (e.g., C), rather than dispersed. Here, by examining Co, Hazen et al. now show that the LNRE distribution also applies to elements, even if they occur not just in concentrated form, but are also dispersed in wide ranging solid solutions. These authors thus show that LNRE distributions can be used to predict how many minerals are yet to be discovered; the editors of Am Min anxiously await the new mineral descriptions that test this hypothesis.

Apatite Mediates Body Fluid Compositions

On page 149 of this issue, Michael Fleet, as a perfect follow-up to Jills Pasteris review in last months issue, investigates the nature of Na and carbonate substitutions in hydroxylapatite; the author finds that these species substitute in a significant way within the hdyroxylapatite c-axis structural channel. A key implication of this finding is that the hydroxylapatite c-axis structural channel may be the key means by which body fluids interact with nanocrystalline bone materials, and so mediate acid-base reactions in biologic systems.

The State of Carbonates in the Deep Mantle

On page 210 of this issue, Solomatova and Asimow calculate crystal structures and relative enthalpies of high-pressure forms of dolomite. They find that a monoclinic dolomite phase has a lower energy compared to other candidate structures, at pressures ranging from 15 to 80 GPa. Their work does not delimit the conditions on which such a carbonate might decompose to other phases, but identifies a potentially important phase for understanding the global C cycle. Their study clearly points to the need for new experiments exploring the structural and phase equilibrium stabilities of comparable Fe- and Mn-bearing carbonate phases.

Ti in the Lower Mantle

On page 227 of this issue, Bindi et al. report a new Ti-bearing bridgmanite-type structure synthesized at transition zone P-T conditions (20 GPa, 1600 C). Their study indicates that Ti may stabilize bridgmanite-like structures at lower pressures and provide clues as to how Ti and other elements are housed within the lower mantle. As these authors note, natural Ti contents are too low to stabilize this new phase in the lower mantle, but it might be stable in certain localized Ti-rich environments. Although not highlighted by the authors, a yet more important implication is that such a component within bridgmanite may be important for explaining high Ti contents in many ocean island volcanic rocks thought to form as lower mantle thermal plumes. This new phase might either provide the source, or control the mineral melt partitioning of Ti and so may be central to understanding what appear to be lower mantle Ti enrichments.

Editors Selections, December 2016

Invited Centennial Articles

Predicting Trace Element Partitioning Behavior

On page 2577 of this issue, Sun-ichiro Karato provides a review of the physical basis for contrasts in partition coefficients. This review attempts to explain a suite of experimental observations, which include the now-familiar Onuma diagrams, so well developed by Blundy and Wood and others, which show how partition coefficients vary with an elements size and charge, but the physical reasons for the dependence of element partitioning on the size of element have been unclear particularly for noble gas elements. Karato developed new models of element partitioning using the models of point defects in minerals and the hard sphere model of silicate melts. Karatos model provides a physical explanation, for example, as to why some phases partition noble gases in proportion to their ionic radii (bridgemanite), while other silicate phases (Ol, Cpx) do not. Karato finds that his models may help better describe and predict partitioning behavior. But Karato also concludes that no physical model can yet satisfactorily predict P-T dependencies of trace element partitioning, and hence that there is still no substitute for a thermodynamic description of partitioning behavior. 

 

Mineralogists at the Forefront of Human Health

On page 2594 of this issue, Jill Pasteris demonstrates why the demarcation between biogenic, synthetic, and inorganic phases is not a simple one, and may erect unfruitful barriers at least in some sub-disciplines of medicine, mineralogy, and materials science. Here, Pasteris examines apatitic biomaterials, which are defined as the synthetic forms of hydroxylapatite (usually nanocrystalline, in some cases inter-bonded with organic molecules) that are used to replace natural bone and tooth materials. Her review illustrates the various ways in which biomaterials are structured and synthesized, with some fascinating insights into how subtle variations in synthesis conditions can tailor the required properties of a mineral to a given biologic function and determine how successfully such materials will operate when implanted in a human system. The take-home message of this review is that mineralogists have much to offer to such research, and argues that biomaterials should be pursued as a major sub-discipline of mineralogical research. As editors, we happily await the continued publication of papers in this field.

 

Articles

Immobilizing Radionuclides With Apatite

On page 2611 of this issue, Rigali et al. review the various ways in which apatite can be used to isolate a wide range of radionuclides from the near-surface environment. These means of radionuclide neutralization include the familiar modes of surface adsorption and partitioning of species into apatite structures. Rigali et al. also review what may be less familiar mechanisms, such as dissolution/(re-)precipitation reactions that are now being used to remediate contaminated groundwater or act as semi-permeable membranes. For example, some recent studies have shown that apatite can dissolve in the presence of U-bearing fluids to re-precipitate as U-phosphate or U-carbonate, and that the addition of hydroxyapatite to contaminated soils may reduce U concentrations in pore waters to levels deemed safe for drinking.

 

A Depth Continuum of Water Release During Subduction

On page 2645 of this issue Gemmi et al. employ cutting edge analytical techniques to determine the structures of two important candidates for carriers of water into the deep mantle: the 11.5 angstrom phase, Mg6Al(OH)7(SiO4)2, and the HySo phase, Mg3Al(OH)3(Si2O7). These phases can form by the breakdown of chlinochlore and so may carry water to depths beyond clinochlore and chlorite breakdown. These phases lack the H-bonded, infinite tetrahedral sheets structure of precursor silicates. The authors find structures with reduced Si-O-Si interconnections and much higher density. Thus, these high-density phases, which can contain between 8-13 wt% H2O, are expected to be stable to much greater depths.

 

Evidence for Mantle Global Warming?

On page 2768 of this issue, Ganne et al. present an analysis of global magmatic temperatures from published data that span the temporal range of 600 Ma to present. Their most dramatic finding is that magmatic temperatures, as measured from whole rock and mineral compositions, record a maximum that falls between 325-125 Ma; these ages are the bookends of the lifespan of Pangea. This time period also coincides with a peak in mantle potential temperature. The authors suggest that these findings support numerical models (e.g., Coltice et al. 2009; Van Avendonk et al. 2016) whereby supercontinent formation results in both thermal insulation, and a disruption of mantle convection, such that increased temperatures temporarily influence supercontinent volcanism. Coltice et al. predict that supercontinent-induced heating should be <100 C; Ganne et al. identify some key targets for high precision thermometry, as a test of the Coltice et al. model.

 

Predicting Mining Accidents, Building Collapse, Etc.

On page 2751 of this issue Jiang et al. provide an update of prior work that indicated that acoustic emissions presage mine collapse. In this new work, the authors present experimental results that confirm that acoustic emissions increase just prior to the collapse of cavities in sandstone and coal. The energy released by such acoustic emissions can be described by a power law, with slightly different exponents for different materials, but the exponents also change with time. A key result then is that collapse of a mine shaft, or bridge or building, may be presaged by both acoustic emissions and their energies. Another fascinating result is that cavity collapse yields a power law with an exponent greater than that associated with crack propagation, the latter being associated with micro-faults and earthquakes.

 

A New Hygrometer and Shallow Magma Accelerations at Etna

On page 2774 of this issue, in a Letter, Perinelli et al. re-calibrate their clinopyroxene-based hygrometer. The original, and new model, are applicable to trachyte or hawaiiite-type basalts. But while magmatically restrictive, the model predicts water contents without precise knowledge of liquid composition, relying on pyroxene components and the P-T conditions of crystallization. They find that at Mt Etna, magmas begin to dehydrate mostly at a <400 MPa and lose most of their water at pressure of <100 MPa. This result corroborates inferences form melt inclusions, and it indicates that eruption triggering, and magma transport acceleration due to dehydration, are mostly relatively shallow processes, at least in the Etnean plumbing system.

 

 

 

Editors Selections, November 2016

Highlights & Breakthroughs

Melts, Mush, and More
On pages 2365-2366 of this issue, Erik Klemetti provides an overview of Paterson et al. (pages 2176-2198 of the October issue), first by outlining the growing consensus that large liquid magma bodies are rare and that long-lived bodies spend most of their time as crystalline mush. Paterson et al., in their study of the Tuolumne Intrusive Complex (TIC), show evidence of massive erosion and re-deposition of early magmatic materials by later magma intrusions. Their field and geochemical evidence indicate that as much as half of an original magma mush may be eroded and either incorporated into a later intrusion, or erupted (if thermally rejuvenated) or migrate downwards, being replaced by more buoyant magmas. This presents challenges for interpreting everything from emplacement dates (if most zircon crystals are recycled from earlier magma batches) to pluton volume growth rates. Klemetti end by noting that at least some numerical models apparently indicate that massive mush erosion and mixing are unlikely; field evidence shows that the unlikely is possible.

 

Invited Centennial Articles

Rhyolite Emplacement From Spherulites

On page 2367 of this issue, Gardner et al. examine geochemical gradients in glass adjoining spherulites in rhyolitic obsidian, to estimate spherulite saturation conditions and growth rates, and by extension, the nucleation rates of spherulites and cooling rates of rhyolite flows. Their modeling of spherulites indicates that nearly anhydrous Yellowstone Plateau rhyolite lavas exhibit post-emplacement (sub-solidus) cooling rates of 0.3 to 1.2 C/day, in the temperature rages of 710 to 430C. This work interestingly complements a paper we highlighted earlier, by Seaman (2013; page 304, v. 98), who examined spherulites in hydrous systems, and showed that they exhibit either blade-like or needle shapes that vary as a function of cooling rate. Together, these studies provide means to evaluate rhyolite cooling rates from spherulite growth ranging from slightly above-solidus to sub-solidus conditions.

 

A Comprehensive Mush Model For Silicic Systems

On page 2377 of this issue (open access), Bachmann and Huber review the genesis of silicic magma bodies. These authors develop a "mush model" to explain silicic systems, both plutonic and volcanic. The model embodies a counterintuitive view that crystal-liquid separation can be more effective at intermediate crystallinities (~50-70% crystals). This is a foreign concept to those studying basaltic systems, as olivine crystals, for example, readily separate from parent liquids at even the smallest of crystal fractions. But these authors argue that for silicic systems, intermediate to high crystallinities (>40-70%) inhibit convective stirring and small pockets of liquid contained within a larger crystal-rich mush can segregate to form eruptible cupolas. A driving force for this model is its apparent ability to simultaneously explain geophysical observations and the incremental growth of large magma reservoirs.

 

A Plagioclase Geobarometer, and Arc Magma Storage Depths

On page 2405 of this issue, Zellmer et al. apply MELTS to calculate maximum An contents of plagioclase for nominally aphyric lavas from the Trans-Mexican Volcanic Belt. They find that for water saturated mafic melts, An contents (predicted by MELTS) first increase with increasing pressure, to about 1.0 kbar, decreasing thereafter as pressure increases (to at least 9 kbar). The authors suggest that maximum An contents reflect the depths at which magmas stall prior to eruption. In their model, nearly aphyric magmas leave their mantle source region with varying amounts of dissolved water. Those liquids with the greatest water contents reach vapor saturation at the greatest depths and partially crystallize as the magma dehydrates. Both crystallization and dehydration serve to increase viscosity, causing magmas to stall and cool further, reaching plagioclase saturation. From this outlook, maximum An contents are a proxy for depths of vapor saturation and pre-eruption staging depths, or the final depths of crystallization for un-erupted fractions.

 

A Volatile-Depleted Chondritic Earth?

On page 2452 of this issue, Jones et al. examine the water and halogen contents of merrillite and apatite crystals from various chondritic meteorites. They find that while volatile contents vary across meteorite sub-types, and even within individual samples, they do not vary systematically with metamorphic grade. It would seem, then, that fluid evolution on chondrites can be quite localized, and that phosphate compositions record a range of processes related to regolith development. One hypothesis is that F-rich apatite grains form within a regolith by interaction with F-rich vapors released from impact-degassed melts near the chondrite parent body surface. To the extent that regolith development is an early solar system process, this mode of genesis implies that chondrites might be strongly degassed prior to their accretion into planet-sized objects. In such a case, Earths inventory of volatiles may be less than inferred from the compositions of non-brecciated chondrite fractions.

 

 

Editors Selections, October 2016

 

Invited Centennial Review

Magmatic Flare-ups Explained
On page 2133 of this issue, Kirsch et al. compare zircon ages along several arc segments that span nearly the entire North and South American Cordillera, from 400 to 80 Ma. They find that age distributions, both within and between arcs, are non-uniform: peaks and lulls are often separated by intervals that fall within a 50 to 80 Ma range. Moreover, while some flare-ups are localized, these inferred (from zircon age dates) magmatic maxima and minima are nearly synchronous for thousands of kilometers in many arcs. They suggest that plate tectonic factors (rather than intra-crustal magmatic differentiation, cooling or transport) may control magmatic input. Some authors have rejected this idea, but perhaps too quickly, having examined smaller-scale spatial and temporal patterns. Rather, by accounting for a lag time between tectonic forces and magmatic responses, correlation coefficients between the two are increased.

 

Graphite In the Diamond Stability Field

On page 2155 of this issue, Mikhailenko et al. combine Raman and FTIR spectroscopic studies to investigate graphite inclusions in diamond, from eclogite xenoliths in the Udcahnaya kimberlite. Their Raman data show graphite inclusions retain a large residual stress, both within the graphite and the immediately adjacent diamond.  This observation, combined with petrographic observations and existing experimental work, leads these authors to conclude that the graphite did not form from diamond on a retrograde path, but rather that graphite existed metastably in the diamond stability field. These authors also find that N in diamond is highly aggregated, which may mean that diamond hosts for highly stressed graphite grains are quite old. This work implies that metastable graphite might persist within at least the immediate reaches of the diamond stability field for perhaps billions of years.

 

Re-hydrated Clinopyroxene as a Magmatic Hygrometer

On page 2233 of this issue, Weis et al. examine the re-hydration of clinopyroxene phenocrysts from various volcanic systems. As with prior studies, they find that hydration and de-hydration reactions are controlled by an equilibrium involving ferric Fe: (OH)- + Fe2+ = O2- + Fe3+ + 0.5H2. Their work indicates that clinopyroxenes phenocrysts may often dehydrate significantly during magma transport and eruption, and so untreated, can only be used to estimate minimum water contents in a magmatic system. These authors argue that pre-eruption magmatic water contents can be obtained using their methods to re-hydrate clinopyroxenes, with the implied argument that ferric Fe contents in clinopyroxene place a maximum on rehydrated water contents. However, ferric iron itself is not a good proxy for magmatic water contents, as it is not always linked to hydrogen associated defects, but may be part of other charge balancing processes (e.g., Na+ Fe3+ vs. 2 Ca2+).

 

Making Granite on the Moon

On page 2312 of this issue, Gullikson et al. conduct partial crystallization experiments to test hypotheses of granite genesis under lunar conditions. They show that lunar-like granites can be obtained by partial melting at low pressures of monzogabbro and alkali gabbro crustal rocks, at least when equilibration temperatures are <1000 oC; but only the monzogabbro parent yields granite magmas in sufficient quantities to segregate from a solid residue. Some of their experiments also yield Fe- and Si-rich immiscible liquids, at T = 1000-1050 oC. But liquid immiscibility, at least for the examined starting compositions, can be rejected as a means to make Th-rich Lunar granite, since Th partitions strongly into the low-Si, Fe-rich liquid. Granites appear to be uniquely abundant on Earth—this work is a step towards understanding their rarity elsewhere and thus for understanding what makes Earth exceptional among neighboring planetary objects.

 

 

A Ti-V-Cr Rich Layer in the Upper Transition Zone?

On page 2322 of this issue, Zhang et al. explore the crystallography of transition elements and intra-crystalline partitioning of these into Wadsleyite. The inside-baseball result: Ni, Co, and Zn avoid the M2 site (in favor of M1 and M3), and do so in proportion to expected Crystal Field Stabilization Energies. Perhaps more significant, though, is that trivalent cations Cr and V (which strongly prefer M3 over M1 and M2), and tetravalent cations, such as Ti, appear to be much more soluble in wadsleyite tetrahedral sites compared to T sites in olivine, apparently because T sites in wadsleyite are larger. The authors suggest that the wadsleyite-bearing mantle (410-525 km) has the potential to absorb more of these elements compared to shallower mantle materials, and plumes that transit the wadsleyite stability field may be affected by this contrast in wadsleyite and olivine solubility.

 

 

Editors Selections, September 2016

Highlights and Breakthroughs

Finding Hydrous Phases on Mars
On page 1925 of this issue, Edward Cloutis provides a review of Farrand et al. (2016; page 2005 of this issue), who show that it is possible to identify minerals by remote sensing, that have been altered by reaction with water. The technique makes use of visible and near infra-red (VNIR) spectra. Farrand et al. found evidence of clay minerals, hematite, hydrated sulfates, hydrated silica, and perhaps even free water in the pore spaces of some soils. As noted by Cloutis, a key value of this study is the demonstration that a coarse spectral resolution can be used to identify a wide range of hydrous alteration products on the martian surface, or other planetary surfaces.

 

Arsenic on the Surface of Fe-hydroxides

On page 1927 of this issue, Michael Schindler reviews the work of Lee et al. (2016; page 1986 of this issue), who use a range of techniques to determine the nature of adsorption of arsenates on Fe-hydroxide mineral surfaces. As Schindler notes, Fe-hydroxides are common in soils and mining waste, and arsenic oxides have a high affinity for Fe-hydroxide surfaces over a wide range of conditions. Lee et al. are the first to analyze an important contaminant that occurs on a highly relevant mineral surface, and in the process are able to develop a model for the reactions that allow contaminants to be sequestered by mineral surfaces.

 

Invited Centennial Review

Oxygen Fugacity of Earth and the Inner Solar System
On page 1928 of this issue, Righter et al. review micro-X-ray absorption near-edge structure (micro-XANES) analyses of V in spinel to estimate oxygen fugacities (fO2) for a wide range of primitive planetary materials. They find that fO2 values are mostly more oxidizing than the sun, and range from 8 log units below to 8 log units above the iron-wustite (IW) buffer. Mars and Earth are more oxidized than chondrites and other meteorite groups, with Earth ranging to much higher fO2 values, presumably because ocean waters and an O-rich atmosphere partially react with subducted materials which are later partially melted. But a more complete understanding awaits samples from Venus and Mercury, which the authors suggest –- with certain legitimacy –- may revolutionize our understanding of planetary evolution.

 

Quantifying Magma Ascent Rates from Bubbles

On page 1967 of this issue, Le Gall and Pichavant use new experimental results to show that average bubble sizes (D), and bubble nucleation densities (BND) vary systematically with magma depressurization (ascent) rates. They find that varying ascent rates, and processes of bubble nucleation, growth, coalescence and outgassing, all yield different trajectories in on a chart of D vs. BND. For example, trajectories are negative for ascent rates of <1.5 m/s, but positive for ascent rates of 1.5 to 3.0 m/s. Their results show that D and BND values from Stromboli Volcano can only be reproduced for ascent rates > 1m/s, followed by ascent-independent bubble growth or coalescence. These results provide a new and potentially powerful tool for uncovering the final storage conditions and changes in magma pressure as a system reaches an eruptible state.

 

Making a Metallic Core – Fast

On page 1996 of this issue Todd et al. present experimental results that show that silicate + FeS mixtures, like their purely silicate counterparts, yield very high permeabilities when under a state of shear stress. The authors hypothesize that small planetesimals may momentarily experience high shear stress states when they collide with other planetesimal bodies in the early solar system. In this way, a small planetary body, too cool to support a magma ocean, might still undergo rapid silicate-melt fractionation, so as to form a metallic core within the 3-5 m.y. as is apparently required by W-Hf isotopic data. Presumably, then, collisions are the driving force of core formation on small planetary objects.

 

Stalled Slabs? Blame Wadsleyite

On page 2085 of this issue, Ritterbex et al. develop a model for dislocation creep in wadsleyite that can be applied to the strain rates and pressure-temperature conditions relevant to Earths transition zone (410-660 km). The mineral wadsleyite, a polymorph of (Mg,Fe)2SiO4, is a dominant phase in Earths transition zone (410-520 km) with olivine stable at shallower depths, and ringwoodite at greater depths (520-660 km). In an earlier study, Ritterbex and others showed that even the easiest of dislocation slip mechanisms in ringwoodite were too inefficient to drive deformation at the base of the upper mantle. The authors extend this work to wadsleyite, obtaining a similar result. These results explain why some subducted slabs stall within or at the base of the transition zone: resistance to subduction may reflect a substantial increase in resistance to deformation as (Mg,Fe)2SiO4 transforms from olivine to wadsleyite, and eventually ringwoodite.

 

Editors Selections, August 2016

 

Highlights and Breakthroughs

Modeling Crystal Structures

On page 1717 of this issue, I. David Brown presents an overview of Wander and Bickmore (2016; page 1862 of this issue), who, as Brown explains, use of spherical harmonics expansion to calculate the energy of aluminosilicate mineral structures. Wander and Bickmore suggest that their approach, provided that bond-valence sums are small, leads to errors that are comparable to those obtained from the (vastly) more computationally intensive quantum mechanical approach. The challenge moving forward is the lack of experimental data that can be used to calibrate molecular models.

 

The Phosphorous-in-Garnet Conundrum

On page 1718 of this issue, William Glassley summarizes new experimental work by Konzett (2016; page 1756 of this issue), which provides a test of the usefulness of P in garnet as a geobarometer. Konzett argues that the lack of high phosphorus garnets in UHP metamorphic rocks reflects later re-equilibration: Konzett synthesized at high pressures, garnets that that are rich in Na-P components; these are then re-equilibrated in an eclogitic bulk composition at lower pressures, where the Na-P components exsolve to form apatite inclusions. This work suggests that a large fraction of apatite in eclogites might originate as a result of decompression-driven dissolution of phosphorus in garnet, and that the preservation of phosphorus-rich garnets may require very rapid upward transport. As Glassley further notes, though, to make use of such experimental results, it will be necessary to analyze Na and P contents at much higher than usual precision.

 

Invited Centennial Articles

The Connection (or Lack Thereof) Between Plutons and Volcanics

On page 1719 of this issue, Barnes et al. provide a detailed analysis of mineral and whole rock compositions of the Wooley Creek Batholith in northern California. They find that the middle to lower structural sections of the batholith are largely crystal cumulates, with little residual melt. Only the uppermost portions of the batholith contain whole rocks that are plausible liquid compositions. Their results imply that plutonic whole-rock compositions are rarely frozen liquids, and thus that plutonic compositions are a poor proxy for arc liquid or arc volcanic compositions.

 

Mineralogic History Through a Garnet Lens

On page 1735 of this issue, Charles Geiger helps to celebrate our centennial by telling the story of mineralogy through the history of studies of garnet. That history stretches back to a time when mineralogy and chemistry were not distinct sciences, and includes: Hays idea of a smallest corpuscle; Goldschmidts ideas of atomic substitutions, controlled by atomic radii; the resulting classification schemes for garnet based on solid solution behavior; and very recent suggestions that, given extensive to complete solid solution, the pyralspite and ugrandite distinctions should be abandoned.

 

Other Notable Articles

Arc Magma Sources Revealed in Zoned Olivine Grains

On page 1807 of this issue, Rowe and Tepley analyze olivine grains from rare absarokite and shoshonite lavas from the Cascades arc. The cores from high forsterite olivine grains have very high Ni concentrations (>6000 ppm) and low Ca (850 ppm), while their crystal rims have low Ni (1000 ppm) and high Ca (>1500 ppm). The authors interpret such zoning as a result of a two-stage history; with the core compositions growing from a harzburgite source that had reacted with a Si-rich slab-derived melt (transforming some olivine to pyroxene in the process). Rim compositions might arise from one of a range of different processes, including late crystal growth from a peridotite-derived melt, or post-eruptive re-equilibration. In the former case, trace element systematics indicate no difference in incompatible element enrichments between the contrasting mineralogic sources, which implies that slab-derived fluids affect trace element concentrations over a broad region, but only locally affect source mineralogy.

 

Mining Fertilizer for REEs

On page 1854 of this issue, Radha Shivaramaiah et al. examine the means by which Rare Earth Elements (Eu, by way of example) are sequestered by phosphogypsym, which is a waste product related to the creation of phosphorous-based fertilizers. REEs are not the great concern they were just a few years ago, thanks to increased global production, but their extraction is nonetheless vital given their use in various technologies, including rechargeable batteries and phosphors in TV and computer screens. Shivaramaiah et al. find that REE are likely to be stored as a nanocrystalline precipitates on the crystal surfaces of phophogypsum, and so should be highly extractable from a material that otherwise fills solid waste landfills.

 

Magmatic Clay?

On page 1906 of this issue, Gammel and Nabelek examine the compositions and paleo-temperatures of fluid inclusions captured by various phases in granite-hosted pegmatites in southern California. Fluid inclusion temperatures are mostly less than 400 C, and range to as low as 70 C—well below the water-saturated solidus of host granite. Their work not only reveals the conditions by which pegmatite minerals precipitate, but observed phase relations and compositions track increases in acidity for hydrothermal fluids, due to precipitation of alkali-rich phases such as lepidolite; these evolutionary trends drive a fluid system towards kaolinite saturation. Clay minerals might thus not be a product of supergene reactions, but instead the very low-T end of a crystallization sequence that begins at magmatic temperatures.  

 

Letter

The nature of invisible gold made visible

On page 1916, Fougerouse et al. present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. Gold incorporation is shown to be controlled by the rate of crystal growth, with slow growth-rate promoting the formation of gold clusters and rapid growth-rate leading to homogeneous gold distribution. This may have consequences on approaches to the economic recovery of Au from refractory ores.

 

Editors Selections, July 2016

Highlights and Breakthroughs

Lunar Regolith Metasomatism
On page 1497 of this issue, John Pernet-Fisher reviews new H-isotope analyses by Treiman et al. (p. 1596 of this issue), who suggest that very low
 dD values are not indicative of unique mantle reservoirs, but instead are related to regolith metasomatism. The lunar regolith has very low dD, due to solar wind and cosmic ray bombardment.  In their new work, Treiman et al. recognize that lunar apatite samples having the lowest dD values co-exist with un-zoned olivine and pyroxene grains, and these unzoned grains may indicate long-term equilibration at very slow cooling rates, which may have allowed such basalts to react with underlying low dD regolith materials, that were in turn heated during basalt emplacement.

New Hydrologic Environments on Mars

On page 1499 of this issue Kathleen Benison reviews Ehlmann et al. (p. 1527 of this issue), who present compelling evidence for alunite on Mars. The mineral alunite is more than just a curiosity. As noted by Benison, we have long known that the ancient martian surface was wet. But less well understood are the compositions of those waters. The acidic nature of the martian surface has not been in doubt, given discoveries of the Fe-rich mineral jarosite. The discovery of alunite, though, not only further confirms that surface waters were locally acidic and S-rich, but Al-rich as well. Alunite is rare on Earth because of the unusual combination of Al- and acidic fluids. But what is rare on Earth might be abundant on Mars. We have yet to take full advantage of our knowledge of rare terrestrial minerals, to better understand the diversity of hydrologic environments on Earth (although work by Hazen and Ausubel, p. 1245 of this volume, is a needed step forward). But such work is perhaps all the more urgent, since as Benison so aptly notes, Mars hydrologic and mineral history is different from, but as complex as, that of Earth.

 

 

Ferric Fe Determinations from the Microprobe? (In Garnets, Yes)

On page 1704 of this issue, Quinn et al. show that the ferric contents of garnets might indeed be obtained by a combination of electron microprobe determinations of total Fe as FeO, and charge balance calculations. The literature is replete with compelling experimental data that show how measured ferric Fe contents are completely uncorrelated with ferric contents determined by charge balance, using electron microprobe analyses of major elements. This failure occurs for a wide range of minerals (e.g., Hawthorne, 1983, Can Min; Dyar et al. 1989, 1993, Am Min). However, Quinn et al. show that garnet may be an exception to the rule. Ferric Fe contents obtained from electronic microprobe analyses and mineral stoichiometry match Mossbauer determinations of the same and that errors of estimation are especially low at high total FeO contents (0.05 at >15 wt% FeO total). Although not specifically explored by Quinn et al., this work opens the possibility of estimating oxygen fugacity from microprobe analyses of metamorphic and igneous garnets.

 

 

Magmas are More Water Rich Than You Think
On page 1691 of this issue, Esposito et al. discover liquid H2O and H2O-CO2 vapor in bubbles of olivine-hosted melt inclusions. Their results show that water moves freely between the vapor and melt (glass) phases and that low water contents in what are necessarily CO2-rich bubbles are not evidence for a dry melt or a CO2-rich source. Instead, low water contents of bubbles may simply reflect post-trapping exsolution loss of water from a bubble into co-existing glass, or the precipitation of water as an annulus about the bubble wall. The annulus can be quite thin, and so optically undetectable, but compositionally molecularly significant. For example Esposito et al. estimate that this easily un-detected water might account for as much as 20-60% of the total water budget of a given melt inclusion, depending upon the bubble/melt-inclusion volume ratio, the size of the melt inclusion, and the initial H2O content of the trapped melt. Their data also show that S contents of melt inclusions might also be underestimated if bubbles and their annuli are not accounted for in the mass balance. Their results emphasize that great care must be taken to obtain accurate reconstruction of volatile contents of sub-surface melts based on analysis of melt inclusions." 

 

 

Recording UHP Metamorphic Reactions

On page 1696 of this issue Ague and Axler propose that minor elements in garnet, such as Na, Ti and especially P, provide a faithful record or prograde and retrograde metamorphic paths, compared to other minor or major elements that more easily diffuse through the garnet structure. In their study, they examined zoning patterns in garnets obtained from high pressure and ultra-high pressure (diamond-bearing) granulites. Their work reveals fine-scale zoning patterns for P that are invisible in Ca or Mg profiles. They suggest that the fine-scale growth patterns observed for P, Ti, and Na record dissolution-precipitation reactions that are driven by prograde and retrograde metamorphic reactions. We appear to have insufficient understanding of metamorphic equilibria to translate P or Ti zonation to specific estimates of temperature and pressure, but experimental studies of such might prove to be gainful employment.

 

 

Disappearing Diomignite

On page 1700 of this issue, Anderson reports on the recent discreditation by IMA of diomignite (Li2B4O7) as a valid mineral species. The lack of evidence for diomgnite's existence negates the inferred role of a Li2B4O7-flux-rich melt in the generation of primary pegmatite textures and rare element oxide mineralization

 

 

Editors Selections, June 2016

Invited Centennial Article

On the rarity of minerals
On page 1245 of this issue, Robert Hazen and Jesse Ausubel consider the fact that of the >5000 known minerals, the vast majority are quite rare. A mere dozen or so make up >90% of Earth's crust, and probably all of Earth, while >99% of Earth consists of less than 100 mineral species. But rarity does not necessarily insinuate triviality; the occurrence of many rare terrestrial mineral species may be yet another unique characteristic in our solar system, along with massive granite-rich continents, abundant water, and life. Hazen suggests that mineral complexity may act as a proxy for other forms of evolutionary complexity. For example, given that 2/3 of known mineral species originate by biologically mediated processes, mineralogy complexity at some threshold value might allow us to identify planets that (necessarily?) harbor life.

 

A zircon geospeedometer for rhyolitic liquids

On page 1252 of this issue, Zhang and Xu present the results of new experiments on the dissolution and diffusion characteristics of zircons in rhyolitic liquids. They find that zircon dissolution depends mostly on temperature, and less so on pressure or subtle contrasts between various rhyolitic liquid compositions. From existing, and their new data, the authors present a model of zircon growth, and find that for hydrous rhyolitic magmas, zircon growth rates range from 0.01 to 1.0 mm/yr, and suggest that zircon size can be used to infer pre-eruption storage times for such systems. Applied to the early Bishop Tuff, they infer that under ideal thermal conditions, zircon crystals, which range to 200 mm diameter in the early Bishop Tuff, experienced residence times of at least 1.4 to 2.2 ka.

 

Rhyolites & Granites – Securely Linked

On page 1268 of this issue, Frost et al. show that the mineralogical similarity of silica-rich systems – mostly quartz + feldspars  -- belies the myriad of processes by which they may be generated. They propose a new classification scheme for leucogranites, based on major and trace element compositions, which preserve a complexity that is not evident from mineral associations alone, although must certainly be reflected within mineral chemistry. Of course, the power of this method is that a geochemical approach provides a means to link mineral-poor rhyolitic systems to sub-solidus granites. Interestingly, these authors show that certain rhyolitic systems do indeed have plutonic equivalents – denied in prior studies because the rhyolites in question were compared to the wrong plutons. Their proposed six-fold (and further) divisions may add some considerable clarity with regard to source and petrogenesis.

 

A Cool Crust (and Subduction?) at 4.4 Ma

On page 1348 of this issue, Harrison and Wielicki examine 4.4 Ga zircons that appear to have formed from nearly water-saturated conditions, at low temperatures of ca. 680oC. But they also show that Tertiary age granites in the Himalaya, formed by under-thrusting of the Indian plate beneath Asia, yield similar temperatures (ca. 660oC). Moreover, these granites share more than a thermal history – both the ancient and more recently formed zircons contain inclusions that yield similar pressures, or 5-15 kbar. Two aspects of these results are remarkable, and highly significant; the first is that if nothing else, the Hadean exhibited, at least locally and for a time, a modern-like continental geotherm, effectively equivalent to a Miocene case (at ca. 25oC/km). Second, to the extent that P-T conditions are indicative, a similar tectonic setting for both systems is implied; and if the association holds, then active plate tectonics – including subduction – occurred from almost the inception of Earths history.

 

Garnet + Clinopyroxene Thermometry

On page 1438 of this issue, Pickles et al. conduct new experiments on equilibrated garnet + clinopyroxene pairs, using synthetic compositions. They find that the partitioning of rare earth elements (REE) is less selective at high temperatures, as might be expected from strain-partitioning relationships. This, and the tendency of trivalent REE to resist re-equilibration at low temperatures, means that REE contents can be used as a thermometer that better preserves peak temperatures of garnet + clinopyroxene co-saturation.

Editors Selections, May 2016

Highlights and Breakthroughs

Re-hydrating the mantle transitions zone

On page 1021 of this issue, Tatsuhiko Kawamoto provides an overview of new work on hydrated oxides, by Nishihara and Matsukage (April issue), which may be stable at high pressures. Kawamoto notes that we have now identified a number of different phases that are potential carriers to of water from the surface to depths of at least 250 km. But we expect that amphiboles and phyllosilicates breakdown by dehydration reactions that release water to the mantle wedge, and so drive arc magmatism. Nishihara and Matsukage (2016) show that Fe-Ti oxides are stable at greater P-T conditions, ranging to 15 GPa and 1500oC, especially for those oxides rich in Ti. As Kawamoto implies, Fe-Ti oxides might actually take on much of the water released during lawsonite breakdown, decreasing the amounts of water going into the mantle wedge, and so increasing that delivered to the mantle transition zone.

Triggering Eruptions From the Deep Crust?

On page 1023 of this issue, Corliss Kin I Sio discusses new results from Viccaro et al. (2016), who show how deep-seated intrusions may have triggered the March–April 2010 eruption at Eyjafjallajkull. These authors measured diffusion time scales that record episodes of magma mixing. Interestingly, they find that olivine samples with higher Fo content cores exhibit longer inferred time scales, based on Fe-Mg diffusion at their rims. Their highest Fo olivine samples (Fo88) yield times scales of about 30 days, while their lowest Fo content core compositions (Fo77) have rims that yield times scales of about two weeks. Viccaro et al. infer that the higher Fo core compositions are derived from greater depths in the volcanic conduit, and so the longer diffusion time scales represent the greater distances that such olivine samples had to travel. Their idea is that the March-April 2010 events were triggered by mantle-derived melts that were fed into the lower crust, with recharge cascading upwards. There is a nice attempt to link magma storage zones using seismic events, but as Corliss points out, the deeper seismic events occur later, not earlier, in the eruptive sequence.  

Electrochemistry of the Deep Earth

On page 1025 of this issue, Abby Kavner reviews our understanding of metal-metal oxide systems, and the contribution of Shofner et al. (2016) in providing the energetics of W-WO. In this work it is shown that W becomes less siderophile as pressure-temperature conditions approach the core-mantle boundary. Kavner plots this behavior as a function of oxygen fugacity, showing that W-WO equilibrium is established at nearly the same fO2 conditions as Fe-FeO at CMB conditions. Shofner et al. also find a new WO2 phase that is stable at high pressures. It is not clear that such a phase is stable, since W does not exist in isolation in the mantle, but in equilibrium with various phases that can dissolve W. So as noted by Kavner, there is still much work to be done to understand how nominally siderophile elements might be partitioned between natural phases at high P and T conditions.  

Presidential Address

Chemistry Trumps Structure in Mineral Classification

On page 1027 of this issue, Peter Heaney discusses two common misconceptions of mineralogy: one, that mineral classification schemes lack a Linnean-like taxonomy, when indeed the chemical systems of Berzelius and Dana exhibit a Linnean tree-like structure; two, that there is not an intrinsic chronology to minerals themselves, an idea now apparently overturned by the mineral evolution work of Robert Hazen and collaborators. Heaney further argues, however, that only the chemical systems of Berzelius and Dana, not their structural or form-based predecessors, could provide a mineral classification tree that could also act as a map of evolutionary chronology. Only the chemical or electromagnetic classifications could provide an effective link to mineral evolution. Heaney uses phosphates as an example of the cladistic nature of a mineral classification tree—a tree that lacks extinction, but involves increasing competition for elements on a planet with increasingly specialized environments. These arguments and the Hazen hypotheses provide a clear path for mineralogists describing new species, as each discovery provides a possible test of ideas that now appear to underpin all of mineralogy.

 

Invited Centennial Article

A Century of Investigating Mineral Structures

On page 1036 of this issue, Angel and Nestola review this history of investigations into crystal structures. Although Heaney suggests that the Berzelius and Dana systems are more useful for classification, Angel and Nestola provide a reminder that mineral structures are fundamental to understanding physical properties and bonding characteristics. Moreover, new advancements in structure analysis provide considerable promise for better understanding everything from geothermometery to deep-mantle water contents. The article focuses however, on the fascinating history of crystallography. The development of X-ray diffraction methods at the beginning of the 20th century was truly one of the most important scientific developments in all the sciences. Through the 19th century Daltons atomic hypothesis was found to be increasingly useful in chemistry, and especially thermodynamics, but it was still no more than a hypothesis, that is until X-ray diffraction demonstrated that atoms were real. The authors further note that analytical methods have sufficiently advanced so that structural interpretations are no longer limited by data, but rather by the assumptions used in common refinement models, e.g., that atoms diffract as un-bonded spherical shells. Development of the theory of bonding, published elsewhere on the pages of Am Min, will be a critical source of future advances.

 

Regular Articles

Helenistic Hammers and Byzantine Anvils

On page 1072 of this issue, Eekelers et al. conduct a petrographic study of iron slag from blacksmith (smithing) sites that date from the 1st to 7th century C.E. The sites are within the ancient city Sagalassos, in southwest Turkey, which is an especially important location as it contains a near-continuous record of technological development of Roman smithing techniques. These authors discovered textural contrasts between two types of smithing techniques—contrasts that would go undetected using only geochemical means. In one smithing process, metals are deformed only at high-temperatures, to make items such as hammers and anvils. But in others, metals are deformed over a range of temperatures and utilize lime or other flux agents to limit oxidation; this latter process produces cutting tools or more finely shaped instruments. The authors also provide a textural index that can be used to decipher which iron deposits in the archeological record have undergone human intervention.

 

Hot and Cold Rhyolite Storage

On page 1222 of this issue, Loewen and Bindeman use oxygen isotope thermometry to compare temperature estimates from nominally hot and cold rhyolites. The "hot" rhyolites are non-water saturated samples from Yellowstone and Iceland, while the "cold" rhyolites are from the Bishop Tuff and elsewhere. The authors employ a single-crystal laser fluorination technique that they find particularly useful for rhyolites, which quench rapidly, at relatively low temperatures compared to basalts. They find that minerals, including near-liquids phases such as magnetite and Cpx, from wet rhyolite magmas rarely yield temperatures that exceed 800oC. Such results are consistent with zircon age dates that indicate prolonged storage at near-solidus conditions prior to eruption allowing for equilibration of oxygen isotope values between minerals. In contrast, hotter rhyolites preserve near-liquidus magnetite and clinopyroxene temperatures at 900-1050oC; zircons record cooler temperatures, but still much higher than for the wetter rhyolites, at >800oC. Loewen and Bindeman show, based on diffusion characteristics, that these hotter systems require pre-eruption residence times of <1000 y, vastly less than the ca. 105 y time spans commonly inferred for highly felsic systems.

Editors Selections, April 2016

 

Editorial

We would like to re-advertise our Editorial on page 497 (on why H-index, and impact factor dont mean what you think they mean, and what Scientists can learn from Baseball), with apologies to the Journal of Crystal Growth. This and a few other crystallographic journals received higher total citations in 2014. We maintain our broader argument, though, that commonly used indices, garnered without context, yield over-simplified, if not meaningless proxies of scientific quality.

 

Invited Centennial Articles

The Structure of Silicate Glasses, and Petrology

On page 753 of this issue, Jonathan Stebbins reviews the structural properties of aluminosilicate melts, with a view to re-engaging petrologists with fundamental thermodynamic properties. Stebbins warns, perhaps rightfully, that the success and widespread use of various petrologic models may dull our curiosity of physio-chemical controls on phase equilibria and diffusion. But an understanding of such can be extraordinarily useful.  For example, Stebbins shows how the solution of Al into a silicate melt converts concentrated negative charges on non-bridging oxygen atoms to more dispersed charges on bridging oxygen atoms; this leads to less clustering of atomic units, with the anticipated result that liquid immiscibility is inhibited by Al-enrichment, as observed by Charlier et al. (2013). In another example, Stebbins illustrates how Si-activity is affected by the mean field strength of dissolved network modifier cations, i.e., Si activity should decrease as field strength increases (e.g., K+ < Na+ < Ca2+  < Mg2+)—a result that as Stebbins shows is mimicked by a(Si4O8) values obtained from MELTs (Ghiorso et al. 2002).  These kinds of insights are assuredly useful, not only in guiding future experiments, or predicting behavior in unexplored systems, but also for informing us when our petrologic models are leading us astray.  

 

Testing plagioclase-liquid thermometry

On page 769 of this issue, Humphreys et al. compare two plagioclase-melt thermometers and the MELTS model, when applied to systems that are known to be at disequilibrium. Building on prior work on forced disequilibrium systems (e.g., decompression or cooling rate experiments), Humphreys et al. demonstrate that thermometers calibrated at equilibrium conditions yield systematically high T estimates at disequilibrium. Their suggestion is that T estimates in natural systems must be considered maxima in the absence of some check on equilibrium. A better test still would be to compare plagioclase + liquid T estimates to an independent thermometer, which should yield T estimates that, if are not the same (for minerals that co-precipitate), then are at least qualitatively consistent with known phase equilibria. In spite of such challenges, however, Humphreys et al. suggest that heating of magmas, through the latent heat of crystallization during decompression (with H2O loss) crystal growth may be a common phenomenon in erupted arc magmas.

 

Nitrogen in the mantle

On page 780 of this issue, Mikhail and Howell examine N speciation and the incorporation of N into diamond. A key observation is that N, having a similar size and charge as C, is incorporated mono-atomically in diamond, rather than as N2 or NH4+, the latter two of which are the most common N species in the mantle. They conclude that it is at least possible that N-rich diamonds may precipitate from a N-poor environment, if the form of N that is available is a monatomic species. Similarly, it is at least possible that N-poor diamonds may form from a N-rich environment, when that environment is rich in N2 and/or NH4+. This implies that N-contents in diamond do not serve as a simple proxy for N contents in the mantle as a whole, and that to understand the N cycle in the mantle, an understanding of more complex N-bearing equilibria is required.

 

Subduction Snapshots

On page 788 of this issue, Stepanov et al. examine P-T paths of metamorphism in the Barchi-Kol terrain of Kazakhstan. They find that peak metamorphism, near 50 kbar and 1000 C, is recorded by a diamond-bearing gneiss, while other samples, from tens to thousands of meters away, yield different and lower peak P-T conditions (24-30 kbar, 710-950 C), comparable to those determined for other samples from the metamorphic belt. More importantly, though, these samples were metamorphosed within a <7 Ma time interval. Thus, their prograde-to-peak-to-retrogade paths provide something of a snapshot of the thermal structure of a subducted and resurrected slab. The authors find a low-T, high-P path typical of subducted slabs, but they also find a near-isobaric heating at 24 kbar (ca. 80 km), that they attribute to the slab coming in contact with overlying asthenosphere. As discussed by these authors, subduction-related metamorphic terranes that do not yield abrupt heating may indicate the subduction of a thicker lithosphere (or perhaps shallow-angle subduction).

 

Pyroxenites in the mantle wedge

On page 808 of this issue Grant et al. conduct experiments where slab-like fluids are reacted with olivine. They find that such fluids yield orthyopyroxene +/- clinopyroxene veins within the olivine hosts. This work shows that metasomatic pyroxenite veins can form over a very wide range of pressures, between temperatures of 750-950 C, and that such reactions explain orthopyroxenite veins found in mantle xenoliths. In this interpretation, these veins may yield our most direct evidence for the evolution and metasomatic effects of subducted-slab inputs into the mantle wedge above subduction zones.

 

The Partitioning of Water and Ocean Island Basalts

On page 876 of this issue, Adam et al. conduct experiments on the mineral/melt partitioning of water for mantle minerals. They find that DH2O is dependent on H2O concentrations in melts as well as tetrahedral Al in pyroxenes. They also find that the partitioning ratio DH2O/DCe varies inversely with the size of M2 site radii in pyroxenes. Since M2 site radii should decrease with increased pressure, then Ce should be relatively more enriched in partial melts formed at greater depths, while H2O would be enriched over Ce at shallower depths of partial melting. These authors conclude that such results support the model of Michael (1995), who found regional similarities between proximal MORB and OIB, but global contrasts between various regions. The authors concur that water contents and incompatible elements in OIB reflect a link between OIB sources and small degree partial-melts derived from local MORB-source materials. 

 

 

150 rare carbon minerals await discovery

On page 889 of this issue, Hazen et al. use the statistical Large Number of Rare Events (LNRE) model to estimate the total number of terrestrial C-bearing minerals. They find that at least 548 such minerals should be present, leaving about 150 yet to be discovered; the authors provide 432 possible species that might eventually be described. Such estimates serve more than idle curiosity. Earth-like exo-planets might be characterized by similar assemblages, which may in turn impact biological evolution. Such predictions also play into the question of whether Earth-like planets form by chance, or are a necessary consequence of early solar system assimilative processes and planetary evolution (if the right bulk chemistry is given). Is Earth mineralogically unique? If so, might it be unique in important or merely trivial ways? The Hazen et al. hypothesis might also implicitly predict the rates at which such minerals may be discovered. These vital questions are in the hands of those mineralogists and crystallographers who study rare and new species—and whose efforts will undoubtedly soon grace the pages of American Mineralogist.

 

 

Editors Selections, March 2016

 

Editorial

On page 497 of this issue, we present an Editorial titled, The most-cited journal in mineralogy and petrology (and what scientists can learn from baseball).

 

Invited Centennial Articles

Fluids and metamorphism

On page 500 of this issue, John Ferry presents an overview of fluid-rock interactions, gained from four decades of research on the Waterville Limestone in Maine, U.S.A. The work provides a fascinating perspective on the lifecycle of geologic hypotheses. In this case, the outcrops in question have been central to the recognition of how fluid compositions are controlled by metamorphic devolatilization reactions, and also to the quantification of fluid flow as time-integrated fluxes, which can be determined by estimating reaction progress in a suite of metamorphic rocks. Early studies though, led to a conundrum: hydrodynamic models indicate vertical, upward fluid flow, but petrologic observations indicated fluid flow in a horizontal, up-temperature direction. Further study of the Waterville Limestone, however, revealed that the observed spatial distribution of reaction progress can be reconciled with upward, down-T flow by explicit consideration of the interplay among solid solution, spatially variable rock composition, and a kilometer-scale gradient in fluid composition during coupled metamorphic fluid flow and mineral reaction. As noted by Ferry, problems related to the up-T-flow-direction model might not have been as readily resolved, had not the problems been identified within the very system that inspired the model in the first place.

 

Magnetic interfaces

On page 518 of this issue, Robinson et al. discuss magnetic substructures that form at the interfaces of cubic and rhombohedral Fe-Ti oxides. The key interfaces are contacts of exsolution lamellae, which deleted yield "intense" remanent magnetization in phases that might otherwise be only weakly magnetic. Such exsolution may result from the well-known oxy-exsolution of ilmenite from titanomagnetite, or reduction exsolution of magnetite from ferri-ilmenite or hematite documented here. Their TEM work shows that the magnetically active regions are focused on compositionally distinct octahedral layers that join the cubic and rhombohedral phases. The authors present mineralogical models to explain these sources of magnetism, with an emphasis on how the interfacial octahedral layers attach to adjacent tetrahedral or octahedral-tetrahedral layers of the adjacent crystals. The implication of this work is that these lamellar structures are the source of significant remanent magnetism in some rock types.

 

Chemical geometry

On page 531 of this issue, I. David Brown asks whether bonds that are strongly covalent are necessarily strongly directional. Central to the analysis is flux theory of bonding, which describes bonding strength as a function of charge divided by coordination number and posits that in a state of static equilibrium, the highest possible symmetry state of a chemical species is preferred (having the lowest potential energy). Brown shows a series of simple equations that allow bond lengths and bond angles to be determined, in the absence of any (nominally artificial) distinction between covalent or ionic bond types. While the model allows one to predict a chemical geometry from knowledge of the chemical formula alone, a remaining challenge is to determine a larger three-dimensional (macroscopic) structure from the individual chemical geometric shapes. The buildup of a 3D structure may cause bond-lengths and bond-angles to diverge from their ideal calculated values, and the induced strains in the molecular building blocks may be relaxed by lowering the symmetry of the macroscopic structure.

 

Dana Medal Paper

Volatile budgets of the bulk Earth

On page 540 of this issue, Marc Hirschmann examines bulk silicate Earth (BSE) ratios of C/H, C/N, and C/S as a signal of combined core-formation, mantle differentiation, late veneer processes, and atmospheric "blow off". Hirschmann shows that it is much more insightful to deal with the three ratios simultaneously; this suggested approach stems in part from their contrasting values compared to carbonaceous chondrite ratios: C/H is lower, C/N is higher, and C/S is nearly equal to that of carbonaceous chondrites. As Hirschmann explains, high C/N and chondritic C/S make it less likely that a low C/H can due to C being lost to space or partitioned into the core. Instead it is suggested that the non-chondritic C/H and C/N ratios of BSE observed today reflect in part the compositions of the materials that accreted to form Earth, rather than reflecting post-accretion, post-late veneer fractionation processes of originally chondritic ratios. And the modern chondritic ratio of BSE C/S?  Possibly a chance result of multiple processes.

 

 

Editors Selections, February 2016

 

Highlights and Breakthroughs & Invited Centennial Article

Wide-ranging Mg isotopes in the deep crust

On page 241 of this issue, Zhang provides a review of Yang et al. (page 243 of this issue, also in Invited Centennial Article) who show that parts of the lower and middle crust exhibit rather remarkable ranges in Mg isotope ratios, despite the fact that shallow crust granitic rocks from the same region are much more homogeneous. The unexpected nature of this result stems from expectations that Mg isotopes hardly budge at high temperatures, but are strongly fractionated by low T processes, such as weathering and carbonate deposition—processes that do not top the list of thoughts on middle or lower crust formation and evolution. Isotopic homogeneity at higher structure levels indicate that processes that lead to granite formation may drive homogenization. But if high-T processes cannot fractionate Mg isotopes, then heterogeneity must result from mixing between the isotopically disparate components that comprise the deeper crust; there may be a great story of middle and lower crust formation, if we can identify what these components are, and how they come together.

 

Invited Centennial Articles

A review of cancrinite minerals

On page 253 of this issue, Gatta and Lotti review the crystal chemistry and structural properties of cancrinite minerals, which are a part of the feldspathoid group. Studies of cancrinite minerals are motivated by their microporous behavior, and their ability to house very large amounts of H, C, S and Cl, due to their characteristic structural incorporation of such components. Here, Gatta and Lotti review experimental studies of how cancrinite minerals respond to increases in T and P. They find that unlike other microporous minerals, cancrinite minerals retain their crystallinity even after being dehydrated. But their review also shows that we still have an incomplete understanding of how the crystal chemistry of this mineral group responds to such changes, which is important for understanding the potential of cancrinite minerals for the storage of solid and radioactive waste, and perhaps also for understanding H, Cl, and S budgets within the crust.

 

Reviews

A history of understanding mafic recharge into silicic chambers

On page 297 of this issue, Bob Wiebe traces the history of our understanding of mafic replenishments into silicic magma chambers. Wiebe applies the term Mafic-Silicic Layered Intrusions , or MASLI, to the field localities that expose mafic-felsic magma interactions. The history itself is fascinating, but there are also important implications to his review. The so-called volcanic-plutonic connection has been a topic of long-standing and recent interest, and as Wiebe notes, MASLI provide a valuable opportunity to explore such. For example, MASLI exhibit stratigraphic sequences that apparently are constructed on a timescale comparable to the time scales of volcanic stratigraphic sections. This being the case, MASLI may provide an even more important target than volcanic rocks themselves for understanding the storage/chamber-specific processes that trigger volcanic eruptions, as well as an examination of any possible dichotomy between the compositions of erupted and un-erupted volumes.

 

Counting magma batches with hornblende (and implications for large eruptions)

On page 328 of this issue, Barnes et al. examine hornblende compositions from the Kuna Crest Lobe (KCL) of the Tuolumne Intrusive Complex, and from the Wooley Creek Batholith (WCB), both in California. Their work shows that the wide T range over which amphiboles precipitate allows these crystals to capture much of the magmatic history of granitic magma bodies. Trace element analyses of these crystals reveal near homogeneity amongst the WCB hornblendes but by contrast, KCL hornblendes vary from sample to sample. These results seem to imply that the KCL was built from numerous small batches of magma, while the WCL batholith, at least syn- and post-hornblende saturation, is comprised of a relatively large and homogenized magma body. The diversity among the KCL amphibole trace elements, despite similar major element compositions, shows that even lacking precise U-Pb age dating, it is possible to demonstrate that a number of magma batches were brought together to from the KCL and that these batches maintained their mineralogical distinctiveness. Moreover, these results imply that a single large magma body of the size of the entire lobe was never available to be erupted from the KCL, but such a body could have crystallized to form the WCB. Amphibole compositions thus provide a potentially powerful means to assess how large eruptible felsic magma bodies are assembled.

 

A hydrothermal signal of magmatic intrusions in the southern Cascades

On page 343 of this issue, Ingebritsen et al. assess the energetic discharge from the Cascades largest hydrothermal system, at the Lassen Volcanic Center in northern California. Their assessment indicates that to produced a steady energy output at the measured value of 140 MW, that this would require the combined cooling (from 800 C to 300 C) and crystallization of a silicic magma intruded at a rate of 2400 km3/year. They calculate that heat transfer occurs over an area of about 5 km2 at a depth fo 4-5 km. Similar mass rates of basaltic magma intrusion are derived from observed CO2 output rates. These long-term intrusion rates can also be compared to observed mappable volcanic output rate of 340 km3/Ma, and leads to a magmatic intrusion:eruption ratio of 7:1. The authors note, however, that there is still much work to be done to understand transients in heat flow and how they are modulated by climate, earthquakes, and the transition from intrusive to extrusive magmatic activity.

 

Thermal maturation of the Bishop Tuff

In page 469 of this issue, Evans et al. argue that Fe-Ti oxides reliably capture at least the late thermal maturation of the Bishop Tuff magmatic system. Prior work cast doubt on earlier T estimates, but as Evans et al. show, the compositions of ilmenites, magnetites, and co-existing glasses track one another rather closely. Roozeboom diagrams illustrate these characteristics, and so provide a compelling, although by no means certain, test of inter-phase equilibrium. These results, together with correlations between compositional parameters and Fe-Ti oxide temperatures, indicate that even if the oxide pairs do not record the correct absolute T estimates from the Bishop Tuff, they probably at least capture the magnitude of temperature changes that occurred post-Fe-Ti oxide saturation. The 100 C interval recorded by such oxides thus represent a minimum T interval over which the Bishop Tuff felsic magmas crystallized, tracking fractionation as T decreases. We still lack a fully quantitative comparison of independent thermometers to test both the absolute values and their relative T ranges; such work, and an analogous assessment of crystal heterogeneity as provided by Barnes et al. (see preceding note) may provide rich rewards regarding the evolution of this very well-known and important "super" (eruption) magmatic system.

 

Editors Selections, January 2016

Highlights and Breakthroughs & Invited Centennial Article

We have three Highlights and Breakthrough articles in this issue.

Spinning Fe in the deep mantle

On page 1, Jeffrey Pigott reviews the issue of whether or not bridgmanite undergoes a high-spin to low-spin transition. The issue is consequent to estimates of mineralogy of the lowermost mantle, based on elasticity and density measurements. Dorfman et al. in the Oct. 2015 issue, appear to settle the issue by showing that in bridgmanite, Fe2+ remains in a high spin state (with unpaired 3d electrons in unfilled orbitals), at reasonable pyrolite-like Fe contents (ca. 10 wt% FeSiO3), but should undergo a transition to a low-spin state (3d orbitals contained paired electrons, leaving some orbitals empty) at higher Fe contents, at 50-70 GPa.

Storing heavy metals in a cement safe

On page 3 of this issue, Associate Editor Maarten Broekman discusses a new study by Khoury et al. in this issue, of natural (Ca,Cd)O, a mineral that occurs only rarely in nature, but as CaO it represents a common accessory in Ordinary Portland Cement (OPC) clinker. Their study of natural (Ca,Cd)O formed by "combustion metamorphism" through ignition of intercalated bituminous lithologies and that was weathered for >100 ka, which formed hydroxides also related to those found in hydrated OPC—and the phases retain their Cd. The work thus implies a reliable means to lithologically sequester heavy metals.

Redox and planetary parentage from spinel

On page 5 of this issue, Stephen Haggerty reviews a study of spinel compositions by Papike et al. (2015). New experiments from Papike et al. reveal greater detail about how V3+/V4+, Fe2+/Fe3+, and Cr partition between spinel and coexisting melt. This new work quantifies the effect of increasing fO2 relative to converting normal spinel to an inverse structure. The gradual change in spinel structure (from >90% normal spinel at IW-1, to <75% normal spinel at QFM+2) means that different kinds of crystallographic sites are available to cations in the melt, and so fO2 has a profound influence on the nature of spinel/melt partitioning. This interplay between temperature, fO2, and redox-sensitive element partitioning means that spinels offer much promise for reconstructing parental magmas and spinel + melt intensive parameters.

 

Invited Centennial Articles

The global nitrogen cycle

On page 7 of this issue, Bebout et al. note that most N is not housed in the atmosphere, but rather a vast majority—ca. 70% (perhaps even more)—resides in the crust and mantle. In their new contribution, they consider various pathways through which N passes from one reservoir to another. Perhaps most interesting is that crucial role of biological processes, in extracting N from the atmosphere, as NH3+ that can later be fixed into mineral phases, thus allowing N to be stored in the crust and subducted into the mantle. Layer silicates are a key means of transferring N from the fluid to the solid state, mostly as NH4+, which substitutes quite effectively for K+ in micas, which are then the main solid-state N reservoirs in the subsurface. Nitrogen is returned to a fluid phase during prograde metamorphism and the decomposition of micas. At least some of these reactions occur with minimal N isotope fractionation, but new isotope fractionation data are needed to better understand these processes.

 

Metamorphic thermochronology

On page 25 of this issue, Matt Kohn reviews the marriage of chronology and thermobarometry in metamorphic systems. Having an advantage in phase complexity, metamorphic petrology has long held the lead over igneous petrology in regard to deciphering pressure-temperature histories, combining these to obtain P-T-t paths. These paths have been crucial in characterizing metamorphic reactions and the tectonic processes that control them. Kohn reviews the most recent means by which P-T-t knowledge is being advanced. Among the many new developments, direct combined thermometry and geochronology appears most compelling. The minerals zircon, titanite, and rutile each provide avenues for simultaneous age dating and thermometry. By obtaining T-t information from each spot analyzed on a given grain, it should be possible to measure crystal growth rates and the energetic potentials that control them, which could add much nuance to interpretations of the P-T-t paths that catalyze recrystallization.

 

Review

K-bentonite chronostratigraphy

On page 43 of this issue, Warren Huff provides a review of global distributions of K-bentonite clays, the altered remains of volcanic tephra. As Huff shows, ancient volcanic tephra provide essential tie points for stratigraphic correlation. K-bentonite-bearing ash layers range from Proterozoic to Cenozoic in age and are shown to preserve high field strength elements or HFSE (which are expected to be immobile during weathering), and these HFSE may provide a tectonic context for the parent volcanoes. Most of the units described by Huff appear to derive from arc-related systems. The global record of arc systems as preserved by K-bentonites may provide yet-to-be exploited and independent record of the waxing and waning of arc activity, related to plate collisional events. As noted by Huff, these may be invaluable for understanding global geodynamics.

Editors Selections, Nov.-Dec. 2015

 

Highlights and Breakthroughs & Invited Centennial Article

The Bond Valence Model and a Theory of Structural Chemistry

On page 2365 of this issue, I. David Brown provides an overview of Hawthorne's contribution "Toward Theoretical Mineralogy", which appeared on page 696 of this volume. As yet, we still do not have a "theory of mineralogy", at least not in the same way that we have a theory of gravity or of biological evolution. As mineralogists well know, thousands of mineralogical structures have been identified, and many more have been synthesized, but just a few dominate Earth and its solar system. But why? As Brown explains, Hawthorne addresses the issue with a concept referred to as the "valence matching rule".  This is a variation on Pauling's rules, where we define the bonding strength of an atom, a quantity closely related to Paulings electrostatic valence bond strength, from the standpoint of both the anion and the cation: given their charge and a typical coordination number a characteristic bonding strength can be predicted. Bonds are most stable when the bonding strength is the same for both the cation and anion. If the bonding strengths differ by more than a factor of two, the bond is unstable, and so unlikely to form; the concept can be taken further, where bonding strength is proportional to Lewis acid/base strength. As Brown states, Hawthorne's new work "elevates the Bond Valence Model to the level of a theory of structural chemistry".

 

Highlights and Breakthroughs

Silicate Liquid Immiscibility

On page 2367 of this issue, Bernard Charlier reviews the experimental work of Hou and Veksler (2015), which appeared on page 1304 of this volume. In the latter work, experimental data are presented that indicate that immiscible ferrobasaltic liquid may be stable at high temperatures—above 1150oC. As Charlier explains, the high T experiments yield silicate liquids that are andesitic, with SiO2 contents in the range of 53-56% SiO2 (and 14-18%FeOt). The experiments thus indicate that high SiO2 contents, as well as elevated FeOt, may be hallmarks of silicate compositions that can reach immiscibility at high T. This then raises the issue of whether tholeiitic liquids can reach immiscibility at high T, since sufficiently high SiO2 contents may occur due to Fe-Ti oxides, but then decreasing FeOt would drive liquids away from the two-liquid solvus. But the new experiments may support recent work indicating liquid immiscibility in the Bushveld.

 

Evidence for Plate Tectonics in The Archean

On page 2369 of this issue, Igor Puchtel provides an overview of Blichert-Toft et al.s geochemical results on komatiites from the Barberton Greenstone Belt in South Africa, which appears on page 2387 of this issue. In earlier work, Puchtel and his co-workers also examined 3.5 Ga komatiites, and discovered a de-coupling of the Sm-Nd and Lu-Hf isotopic systems. The decoupling can be explained by internal mantle differentiation, i.e., partial crystallization of a magma ocean. However, as Puchtel explains, Blichert-Toft et al. find even greater degrees of de-coupling at 3.5 Ga, to an extent that cannot be explained by internal mantle differentiation alone. One possibility is that subducted pelagic sediments contribute to the isotopic signature of some 3.5 Ga komatiites. Such sediments would have little zircon, and so contain high Lu/Hf, which over time would develop high eHf isotopic signatures.  This implies that a modern-like form of plate tectonics was operative before 3.5 Ga.

 

Roebling Medal Lecture

How Trace Elements Change Their Stripes

On page 2371 of this issue, Wood and Kiseeva examine how elements under certain conditions are lithophile, but under other conditions become chalcophile, and vice versa.  They show that the sulfide/silicate liquid partitioning of chalcophile behavior, while generally linear with respect to element valency (slope) and T and P (intercept), that the linear coefficients depend upon how a given element interacts with oxygen. This interaction can be quantified as the difference in lithophile and chalcophile properties of a given element, and FeO. Their new experiments indicate that lithophile or chalcophile behavior can depend on the FeO content of a silicate liquid, and that elements that normally behave as lithophile, may become chalcophile at either very low or very high FeO contents of coexisting silicate melts. Such behavior implies that elements such as U or Th, under reducing conditions and with the addition of sufficient S, might partition into a growing metallic core, rather than a silicate mantle, which in turn could affect the powering of a geodynamo.

 

Invited Centennial Article

Petrology on Mars

On page 2380 of this issue, McSween reviews the petrology of Mars. This review reveals some interesting and important features: First, and unsurprisingly, the martian surface is dominated by lavas, volcaniclastics, and ultramafic cumulates. But among these, alkalic rocks are common in the more ancient terranes, but are mostly absent from younger terranes that are dominated by tholeiitic compositions, which suggest some manner of temporal evolution. But highly evolved compositions, such as granites, are effectively absent. Some silica rich sediments have been observed that were probably created by hydrothermal processes. There is some evidence for metamorphism, in the form of metabasalts and serpentinites, indicating low-P hydrothermal processes. But until such rocks are analyzed directly at the surface, metamorphic processes are still speculative.

 

 

Oxidizing Accretion and Implications for Si in the Core

On page 2739 of this issue, Georg and Shahar present geochemical models of accretion and simultaneous core formation on Earth, and examine the implications of oxidizing conditions and its effects on metal-silicate partitioning. They find that greater FeOt contents in the Bulk Silicate Earth (BSE) imply greater amounts of FeO, and thus more O, in the resulting core. This O content in turn affects the partitioning of Si into the core. With initial FeO in the BSE of 11 wt%, Si has a  maximum concentration of ~2 wt% in the core; this maximum Si content increases to ~3% when initial FeO of the BSE increases to 15 wt%. An implication of oxidizing accretion is that Si partitioning into the core is too weak to greatly affect Si isotope contrasts between the core and mantle. So under oxidizing conditions, the mantle should have a near-chondritic Si isotope signature, unless pressure or temperature play some role in affecting isotopic partitioning. A tentative conclusion, then, is that solar nebula processes may be responsible for generating Si isotope contrasts in the inner solar system.

 

 

Editors Selections, Oct. 2015

 

Highlights and Breakthroughs

Dating Young Mafic Volcanics with U-Th Geochronology

On page 2017 of this issue, Bernal provides an overview of a new approach to U-Th geochronology, as provided by Wu et al., beginning on page 2082. The new study provides a means to date young, silica undersaturated volcanic rocks, through the analysis of baddeleyite, which appears to crystallize largely in vesicles in such rocks. The age dates, obtained for lavas from Campi Flegrei, near Naples, Italy, are similar to K-Ar ages, and are thought to -- at least mostly -- reflect eruption ages, although some age dates may reflect late-stage pre-eruptive crystal growth. As Bernal notes, this advances open up new avenues of research and possibly much better understanding of the timescales of crystallization and eruptive emplacement of mafic volcanic systems.

 

Invited Centennial Article/Outlooks in Earth & Planetary Sciences

Spinels as indicators of redox history and planetary parentage

On page 2018 of this issue, Papike et al. use partial melting experiments to examine the crystal chemistry of spinels, and the effects of valence state on the partitioning of V, Fe, and Cr. They show that increasing fO2 from IW – 1 to FMQ + 2, produces an increase from a mostly normal spinel structure (>90%), to a structure containing up to 25% inverse spinel. This structural change affects the relative compatibilities of V3+ and V4+, since the inverse structure can incorporate larger fractions of the more oxidized species. Their work allows the use of V4+/V3+ ratios to be more quantitatively related to parental parentage, since V4+/V3+ ratios vary strongly between terrestrial, martian, and lunar samples and so can be used to reconstruct contrasting fO2 histories.

 

Diffusion Timescales: 1D vs. 3D Modeling

On page 2026 of this issue, Shea et al. measure diffusion profiles in olivine, to assess the timescales of magmatic processes. Their work shows that results obtained from random 1D profiles can yield timescales that differ from the 3D case by factors that range from 0.1 to 25. Moreover, 1D profiles, even when corrected for mineral anisotropy, still differ from 3D results by factors ranging from 0.2 to 10. The authors find that a set of selection criteria, including size, crystal shape and diffusion plateaus can be used to increase accuracy to 5% and decrease precision to 15-20% relative, when results from as many as 20 traverses from such selections are averaged.

Phosphorous in Olivine

This month, two papers in the Special Section: Olivine, feature studies of P in this nesosilicate. On page 2053 of this issue, Watson et al. investigate P diffusion in olivine and molten melt, concluding that anomalously high P concentrations can develop in olivine when P is built up in a melt boundary layer during growth, but this mechanism requires growth speeds approaching those relevant to dendrite formation.  Alternatively, high P contents of olivine may be a consequence of "growth entrapment" of a high P near-surface region in the olivine lattice, which can occur at more modest growth speeds.  Either way, cooling must occur on a time scale of months to preserve delicate P zoning features. Then on page 2043 of this issue Fowler-Gerace and Tait show that very high P concentrations are present in olivine from a pallasite meteorite (a class of meteorites where olivines are entrained in a metallic matrix). Their textural and compositional analysis indicates that high P regions in these olivine grains formed before complete cooling of the metal matrix but after the olivines were rounded. They relate the high P overgrowths to very rapid crystal growth, recording an impact event on the pallasite parent body.

 

Origin of High 3He/4He Basalts

On page 2066 of this issue Garapic et al. present new analysis of high 3He/4He basalts from Hawaii, Samoa, Iceland and Galapagos. Their study shows that these lavas have high Ti contents and enriched Pb-isotopic signatures. The Ti contents are far too high to be obtained by even very low degree partial melting of a primitive, let alone a depleted mantel source. But the high Ti contents and enriched Pb-isotope signatures are plausible if at least a portion of the source regions are mixed with recycled MORB crust.  But in their view, the high 3He/4He ratios are not related to the subducted MORB material. Instead, they argue that high 3He/4He is indeed derived from a primitive mantle source, but that such a source exists only in a form that has been mixed with recycled eclogites derived from subducted MORB.

 

A new hygrometer for volcanic systems

On page 2172 of this issue, Waters and Lange present a new calibration for a plagioclase-liquid hygrometer. What's especially important in the present update is that the authors present rigorous tests of the new model, using data from five different studies that are not used for calibration, including anhydrous, water-undersaturated, and highly viscous melt compositions. These tests indicate that when temperature is known, water in plagioclase-saturated liquids can be predicted to 0.5 wt%.

 

 

10-fold Symmetry

On page 2340 of this issue, Bindi et al. report the first mineral with decagonal (10-fold rotational) symmetry, decagonite. This represents only the second quasicrystalline mineral to be recognized by the IMA, after icosahedrite, which instead exhibits a fivefold rotational symmetry. Synthetic quasicrystals are often found in three-component alloys, many of which contain Al, the dominant component of both quasicrystalline minerals. The existence of these quasicrystalline minerals through geological times is of strong relevance to discussions concerning the stability of quasicrystals in condensed matter physics.

 

Affirmative Action Needed for Minerals

On page 2344 of this issue Robert Hazen et al. give a statistical analysis from data drawn from large mineralogical databases to predict that at >1500 mineral species remain to be discovered, representing about a quarter of the >6000 species so far described. Their model shows that the as-yet undiscovered minerals are not uniformly distributed across the periodic table, and that sociological aspects of mineral collection, e.g., perceptions of beauty, distinct color, etc., strongly affect the chance of reporting. Obvious implications are that a new emphasis on hypothesis-driven discovery is needed to provide a distribution of known species that more accurately reflects natural variation.

 

 

Editors Selections, August-September 2015

Editorial

Recently, it was suggested that data are eternal. On page 1657 of this issue, we consider to what extent data or ideas stand the test of time, and how such views impact what is, or should be, published.

Outlooks in Earth and Planetary Materials/Invited Centennial

Beyond Equilibrium

On page 1659 of this issue, Carlson et al. show that many metamorphic systems exhibit substantial evidence for disequilibrium reaction paths and phase assemblages. Carlson et al. acknowledge that much has been learned from using equilibrium states as a benchmark for understanding natural phase assemblages. But they also show, for example, that mapped metamorphic isograds can be displaced from expected positions by 1 or 2 km if equilibrium conditions are assumed. Similarly, solid-state mineral reactions and sequences, and mineral zoning, can lead to erroneous interpretations, unless kinetic factors, rather than equilibrium states, are used to analyze reaction paths. The authors demonstrate how disequilibrium effects can be used to better understand the response of metamorphic rocks to changing P-T conditions during metamorphism.

A Model for Granite Genesis and Pluton Development

On page 1762 of this issue, Lee et al. present a field study that indicates that support the Bachmann and Bergantz (2004) model of granite melts being extracted by compaction/hindered settling processes. In their model, basaltic magmas differentiate by fractional crystallization to form intermediate composition daughter liquids. These daughter products differentiate further rising upwards together with mafic enclaves, but without significant crystal-liquid separation. Once emplaced into the middle or upper crust, compaction/hindered settling processes eventually lead to the expulsion of a felsic magma, which creates a cap on any undifferentiated intermediate composition parent liquids, or the mafic crystalline residues that form as a consequence of the melt segregation process. What is particularly interesting in their model is that if the expelled felsic melt fraction is small, and especially if some residual melt is retained, the restite portions of the pluton may not be greatly different in composition compared to the intermediate composition parent. This model thus explains why some plutons can be successfully modeled as a liquid, and yet contain the textural characteristics of a cumulate.

Extracting High Silica Rhyolites from High Silica Granites

On page 1778 of this issue, Graeter et al. examine the textures and compositions of quartz and feldspars within lithic fragments of the Kaharoa eruption of the Taupo Volcanic Zone, New Zealand. Their work shows that many different quartz grains, with different magmatic and resorption histories, are brought together in preferred orientations and in clusters. They interpret the development of such structures to represent crystal accumulation processes that occur by hindered settling and compaction, which shows that at least some high silica plutonic rocks form synchronously with the eruption of high-silica crystal poor rhyolites, through the settling and compaction of multi-stage quartz and feldspar crystals. Their data indicate total crystallinities of 25-60% at the stage of high silica melt extraction.

The Makings of a Apatite Magmatic Hygrometer

On page 1790 of this issue, McCubbin et al. provide new experimental data that bring us a step closer to calibrating a magmatic hygrometer based on apatite-liquid equilibria. Their work shows that a key flaw in current hygrometry methods relate to very uncertain conditions of apatite saturation. The authors instead recommend that hygrometry may be more precisely accomplished through the use of OH-F exchange equilibria, and knowledge of the F content of the bulk system or the parental magma that was saturated with apatite. So in effect, precise apatite hygrometry would appear to require precise melt fluorometry, as a prerequisite.

Containing Arsenic in Mine Wastes

On page 1803 of this issue, Paktunc et al. make a potentially important advance in understanding how to mitigate As releases upon weathering of mine waste. Arsenic is commonly held by scorodite, but in Ca-poor environments scorodite can dissolve, releasing As in the process. Paktunc et al. provide experimental evidence to show that in a Ca-rich environment, scordite dissolution can be accompanied by formation of arseniosiderite, which has a very low solubility at pH conditions of 4.5 to 10.5. Thus, the effect of lime neutralization of mine wastes should simultaneously help to stabilize arseniosiderite and minimize As release.

Zircons as a Record of Paleoseismicity

On page 1834 of this issue, Kovaleva et al. investigate zircon microstructures in association with pseudotachylite in the Ivrea-Verbano zone of the southern Alps. Their zircons are from ultramylonites that contain pseudotachylite veins, the latter of which have provided one of the few geologic means to identify a paleoseismic zone. Their work shows that planar fabrics in zircons are not restricted to meteorite impact sites. Planar fabrics can also be developed by earthquakes, provided that differential stresses and strain rates are sufficiently high. This association opens the possibility of directly identifying and dating paleoseismic events in the deep crust.

Returning water and K to the Mantle (in minerals that actually exist)

On page 1848 of this issue, Han et al. examine the compositions and stabilities of phengite and the omphacite precursors from which they form. In many other issues of this journal, we have reported on structures synthesized in isolation at high pressure that may be candidates for containing water or K in the mantle. The obvious problem is that many such structures may be irrelevant to Earths mantle, as such investigations rarely examine the issue of thermodynamic stability within a peridotite or pyroxenite bulk composition. In this work, Fan et al. show that natural omphacite from the Qiadim UHP province of NW China, can contain >1 wt% K2O and as much as 10,000 ppm H2O at pressures exceeding 6 GPa. Subducted eclogites may thus provide a means of recycling water and K2O back into the convective mantle.

The Source of Water in Olivines Transported in Kimberlites

On page 1912 of this issue, Baptiste et al. conduct experiments to examine possible hydration reactions between synthetic olivine and a model kimberlite-like liquid. In their experiments olivines equilibrated with such liquids show no evidence of hydration. The authors suggest that the presence of CO2 in their model kimberlite liquid serves to catalyze olivine growth, but also decreases the fugacity of H such that less (or no) H is incorporated into the seed olivine crystals. They conclude that since diffusion of H into olivine appears to be quite limited, that high OH contents in mantle-derived olivines are likely to reflect water contents of the mantle source, rather than a more-shallow liquid-crystal reaction.

Finding the Right Zircons

On page 1952 of this issue Davies et al. examine zircon crystals from Scourie dikes of NW Scotland. Their use of Raman spectroscopy and trace element analyses by electron microprobe, allow them to identify areas of zircons that have been altered by fluids. In this way, they are able to identify zircon areas that better reflect original magmatic compositions, which in this case reveal very low O18 values. They suggest that such non-destructive techniques should be regularly applied prior to SIMS analysis, so that unaltered zircons may be more easily targeted.

Carbonates and Fe-Oxides at the Bottom of the Mantle

On page 2001 of this issue, Merlini et al. Provide the first single-crystal evidence of a tetrahedrally coordinated carbonate structure (Mg2Fe2C4O13, with C[IV] in C4O13 chains) containing trivalent ion (Fe3+) at P = 135 GPa and T = 2650 K. These conditions approximate the P-T conditions of the top of the D layer. Their experiments, involving the decomposition of magnesite-siderite mixtures, are important in that they reveal the effect of adding Fe to the mineralogy of the deep mantle. In the same experiments they detect a new ferric iron oxide, Fe13O19, with a new stoichiometry and a new structure. Due to their densities, these phases may sink to near the bottom of the mantle. The implications of these results are far-ranging, affecting our understanding of the mineralogy of the deep mantle, the D seismological anomaly, and deep Fe-C cycling and redox-equilbiria.

Editors Selections, July 2015

Highlights and Breakthroughs & Invited Centennial Articles

Isotopes at a small scale

On page 1333 of this issue, Steve Parman provides an overview of Valley et al. (p. 1355), where they use atom probe tomography (APT) to analyze isotope ratios in zircon crystals. As Parman explains, APT allows for nano-scale resolution of isotopic variations. For example, Valley et al. find that Pb and Y in some zircons occur in clusters, and that such inhomogeneity reflects diffusion into amorphous regions of the crystal formed by radiation damage. Such diffusion requires reheating, which can be dated using the isotope ratios of the cluster. APT thus provides a way to extract complex geologic heating and cooling histories from single crystals, as well as a means to confirm closed-system behavior at larger scales, relevant to the interpretation of bulk SIMS analyses.

Shrinkage Bubbles as the Main Host of CO2

On page 1335 of this issue, Claudia Cannatelli reviews the work of Wallace et al. (2015; April issue) who demonstrate the significance of shrinkage bubbles in estimating volatile contents from silicate melt inclusions. Their work shows that the vast majority of CO2 (up to 90%; 75% on average) that is contained in melt inclusions prior to quenching is lost to shrinkage bubbles. This post-entrapment lost impacts interpretations of pre-degassing volatile contents, as further means that pressures of entrapment are probably often much greater than may be inferred from H2O-CO2 contents derived from silicate contents alone. These findings may move us toward a resolution of the contrasts in P estimates derived from melt inclusions based on fluid saturation and the oftentimes greater pressures obtained by mineral-melt equilibria.

Invited Centennial/Outlooks in Earth and Planetary Materials

We have two Invited Centennial Articles this issue: On page 1337 of this issue John Brady considers the future of the Mineral Sciences. Brady notes that while undergraduate enrollments in the geosciences has increased, graduate enrollments and degrees awarded have declined, as has inflation-adjusted total spending in Earth Sciences research. Brady urges members of MSA to better promote Mineralogy, to our students and colleagues, as a matter of advancement in the sciences, and as a mater of public good. Then on page 1341 of this issue, Kirkpatrick et al. provide a review of NMR and computation techniques that together reveal how cations and organic matter interact with fluids and mineral surfaces. Their review shows how computational methods provide valuable molecular-scale interpretations of NMR observations.

The Color of Diamonds

On page 1518 of this issue, Howell et al. examine the coloration of gemologically valuable pink Type 1 (N-bearing) diamonds. They note that pink diamonds have been found in association with deformation twins. So a key question is whether the twin-planes themselves are responsible for the diamonds hue. Their EBSD analyses of Group 1 (low N) and Group 2 (high N) pink diamonds, however, reveals a lack of twin structures in the former. These authors entertain the possibility that the pink Group 1 crystals were originally twinned, but then were later untwinned. If so, this implies that their pink colors results from defects that originate by the process of twinning, rather than from the twins themselves. Brown diamonds also lack twins and their color banding is thought to be generated by slip planes. Their analysis of N contents further constrains residence times, albeit quite broadly, since diffusion is highly sensitive to T. For example, Group 1 N concentrations are low, but highly aggregated, implying either long residence times (> 3 Ga) at 1200 oC or shorter residence times (200 Ma) at 1300 oC. In either thermal scenario, mantle residence times of Group 2 diamonds (higher N, but less aggregated) is considerably shorter, at <1.5 Ga at 1100 oC or <30 Ma at 1200 oC.

Granitic Rocks at the Lunar Surface

On page 1533 of this issue, Seddio et al. examine the silica polymorphs from granitic samples from the Apollo 14 mission. Their investigation of hackle patterns is suggestive of decrease in molar volume on cooling, and hence the initial precipitation of a high molar volume polymorph. Their volume contraction calculations lead them to surmise that silica initially crystallized as tridymite or cristobalite, which appears to be confirmed by the presence of up to 0.7 wt% TiO2, which is more compatible in the high T polymorphs. These authors conclude that these granitic samples formed as volcanic rocks, and that similarly formed rock types may explain felsic domes elsewhere on the Lunar surface.

High Field Strength Elements are Fluid-Mobile

On page 1600 of this issue, Tanis et al. investigate the solubility of Nb in rutile-saturated fluids. Their work shows that Nb solubility in fluids increases not only with increasing T (as the rutile effectively dissolves) but also with increasing chloride concentrations, although rutile always contains more Nb than coexisting fluids. Their results are important in that they may explain HFSE enrichments in subduction-related gabbros. One hypothesis is that HFSE enrichments may derive by dehydration of serpentinites. This idea is bolstered by this work given that serpentinite-derived fluids may be chloride-rich. Natural fluid compositions might also be just right so as to allow HFSE to be mobilized by the dehydration of blueschist in the formation of eclogite. HFSE are thus perhaps much more mobile than previously thought.

Editors Selections, May-June 2015

Highlights and Breakthroughs & Invited Centennial Articles

Solving the Dolomite Problem

On page 1017 of this issue Carlos M. Pina provides an overview of Rodriguez-Blanco et al. (page 1172), which elucidates the transformation kinetics for the growth of dolomite. Dolomite is a common mineral, but uncommonly resistant to our uncovering the nature of its formation. While perhaps not completely solved, the classic dolomite problem is at least now better understood. Rodriguez-Blanco et al. show that the process begins with aqueous precipitation of a Mg-Ca amorphous phase, which transforms to proto-dolomite at 25-200 C through a spherulitic growth mechanism. Proto-dolomite then transforms to dolomite through an Ostwald ripening-like process, at temperatures >140 C.

Presidential Address

The Worlds Most Important Mineral?

On page 1033 of this issue, John Hughes provides a paean to the mineral apatite, which may have the reader convinced that apatite is the most important naturally crystalline substance on Earth. Most readers will be quite familiar with the use of apatite for various age dating studies. But its relevance to the future of agriculture is rather compelling, as apatite is now a major source of P for fertilizers, and investors are curious about the possibility of peak P (like the one in oil that may be plural). And then of course, there are applications to the sequestration of radioactive nuclides and its importance for understanding bio-skeletal materials and its action as a reservoir for Ca and P in biogenic systems. So, is there a more important mineral? Well, maybe pyroxene.

Material Transport in the Lower Mantle

On page 1053 of this issue, Karki et al. use density functional theory (DFT) to examine grain boundaries in MgO, whose polymorphs are important in the lower mantle. Their results show that native defects and impurities (Ca, Al) segregate to grain boundaries, and that the segregation increases as pressure increases. The enthalpies of migration are also estimated to be lower for impurities residing at grain boundaries, which implies that material transport should be increasingly be dominated by anisotropic grain boundary processes, compared to bulk diffusion as depth increases in the lower mantle.

Building Planets

On page 1098 of this issue, Righter provides a review of aspects of metal/silicate partitioning and the implications of such for understanding core formation and residual silicate mantle compositions (see also Introductory Remarks by Rushmer and Watson on page 1093). Righters review shows that distribution coefficients for moderately siderophile elements vary smoothly with P and T when a thermodynamic approach is used for modeling, which leaves out the apparent discontinuities observed in different approaches. When such models are applied to Mn, Cr, V, Nb, or Ta, show that rather different courses of history are obtained, depending upon whether the modern shallow mantle concentrations of such elements are affected by metal/silicate partitioning alone, or are also affected by deep primitive layers or contrasts in lower and upper mantle mineral/liquid partitioning. If these elements are controlled solely by metal/silicate partitioning, a hotter or more reduced early earth is required. Finally, metal/silicate partitioning of S, C, H, and N may ultimately control the content of these elements in the mantle, and the nature of the light element in the core -- there is much additional work required to fully understand these multicomponent systems and their effects on siderophile element partitioning.

D/H Ratios in Earths Mantle

On page 1182 of this issue, Wang et al. examine deuterium (D)/hydrogen (H) ratios in synthetic silicate glasses through NMR. They discover that inter-molecular contrasts in D/H ratios within the glasses vary to a far greater extent than can be explained by entropy contrasts in the molecular species. Instead, it appears that D prefers sites of low partial molar volume. Wang et al. refrain from predicting melt/fluid partitioning of D/H during early Earth magma ocean conditions, but the qualitative implications are clear: as P increases, Ds preferences for lower molar volume sites means that in a water-saturated magma ocean, D should be preferably held within a silicate melt, while H is preferentially partitioned into a fluid phase. D/H ratios upon mantle dehydration are thus expected to be much lower for the early Earth, compared to today, where cooler upper mantle conditions translates to lower pressures of melting and possibly volatile saturation.

Immiscible Silicate Melts

On page 1304 of this issue, Hou and Veksler show the results of experiments that test whether Fe-rich silicate liquids may be immiscible at geologic temperatures (1150-1200 C). Their work verifies some previous work, which shows that silicate liquid immiscibility may be viable in some geologically relevant high-Fe systems, although implied consolute points derived from these experiments tend to be higher than in some previous studies. In any case, this work provides greater impetus to the growing view that silicate liquid immiscibility may be an important mechanism of magma differentiation for certain natural compositions.

Stishovite on the Moon

On page 1308 of this issue, Kaneko et al. report the first example of a high-pressure silica polymorph from a lunar sample collected as part of the Apollo missions. They suggest that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the near side of the Moon. The authors suggest that re-examination of Apollo samples may allow us to identify and date specific impact event on the Moon, and thus it may be valuable to revisit lunar samples from a high-pressure mineralogy perspective.

Editors Selections, April 2015

 

Highlights and Breakthroughs & Invited Centennial Articles

Constraints on Martian Habitability
On
page 669 of this issue, Javier Cuadros reviews a new open access paper by Bristow et al. (page 824) on martian clay minerals. As Cuadros explains, compared to remote infrared data, new XRD measurements of clay minerals and Fe-oxides from the Curiosity mission may complicate our picture of water retention and water-driven reactions on the martian surface. The new data indicate low water/rock ratios, but push to later dates the time interval for water-rock interaction on the martian surface. Bristow et al.'s study also indicates that the weathering processes that created martian clays occurred over periods of thousands to hundreds of thousands of years, placing a minimum time interval on which life could evolve.  Bristow et al. also point to H2 production in redox reactions may provide at least a potential energy source for biologic activity.

 

Physics and Mineralogy

On page 671 of this issue Wilfried Schranz provides an overview of our invited Centennial article in the February-March issue by Ekhard Salje (page 342).  Salje shows that physicists and mineralogists are pursuing non-intersecting lines of research in regard to microstructural mineral features, such as twins, holes and tweeds (the latter being the precursors to twin structures). Physicists are making use of such defects to produce new electronic devices, but without the benefit of understanding of the mineralogical context in which such defects occur. In the mean time, however, breakthroughs with regard to super-conductivity and ferro-elastic behavior has led to new physics-derived insights that can aid mineralogists in understanding transport and deformation properties of natural materials.  

 

The Importance of Bubbles

On page 672 of this issue Jacob Lowenstern provides a review of Moore et al. (page 806), who examine the effect of vapor bubbles on estimates of volatile concentrations in melt inclusions (MI). Bubbles are common in MI and are thought to occur as MI are cooled and decompressed along an isochore (with minerals walls serving as a fixed-volume container). Moore et al. show that bubbles contain a significant fraction of low-solubility volatiles, such as CO2, comprising anywhere from 40-90% of the total CO2 budget for MI. For CO2-rich systems, the volatile budgets of bubbles are ignored at the risk of confusing pre-eruptive degassing conditions with post-entrapment CO2 melt/vapor fractionation.

 

Mineralogy and a sustainable future

On page 674 of this issue, Alexandra Navrotsky provides a remarkably fitting overview of how Mineralogy segues into various disciplines of interest to society. For example, she compares materials science advances in vapor deposition and the precipitation of phases from the early solar system. As another example, she notes how mineralogists have developed high-P equations of state that are useful to materials scientists (to understanding bonding), but that advances in understanding perovskite defect structures in the materials sciences may also provide new analog structures for studying water sequestration in deep planetary interiors. As she concludes, partnerships between mineralogists, materials scientists, and industry are needed to facilitate this natural research synergy, and should enable advances on the most important problems, and place our society on an environmentally sustainable footing.

 

A Theoretical Approach to Mineralogy

On page 696 of this issue, Frank Hawthorne presents a fascinating approach to determining why some atomic arrangements are stable while others are not. An intriguing aspect of this work is the application of bond topology to structural units in minerals. For example, oxysalt structures can be divided into a strongly bonded structural units, which act as Lewis bases, and weakly bonded interstitial units, which act as Lewis acids, and the corresponding basicities and acidities can be calculated. Stable structures are those where Lewis acidities and basicities match. As an example, Hawthorne shows how this theory predicts stable stoichiometries for MgN(PO4)(OH)m; only when N=2 and m (less than or equal to) 2 is the valence-matching principle satisfied, and these are the only stoichiometries observed in minerals. For oxysalts in general, Hawthorne also shows why stoichiometries with octahedrally coordinated to tetrahedrally coordinated cations in the range 0.25 to 4.0 are stable, whereas stoichiometries outside this range do not occur. This theoretical approach to Mineralogy has the potential to explain atomic arrangements and chemical compositions of terrestrial planetary objects, and with further development, may provide an understanding of the relation between atomic arrangements and their thermodynamic properties.

 

Zircons Record Exhumation, not UHP Conditions

On page 897 of this issue, Kohn et al. model Zr budgets and zircon growth along common P-T paths under a range of P-T conditions. Kohn et al. find that zircon growth and modal abundances can best be predicted by considering decreasing Zr solubility in co-existing, low-Zr phases. Kohn et al. find that most zircon saturation and growth is expected to occur upon exhumation and cooling, rather than under ultrahigh-pressure (UHP) metamorphic conditions. Identifying zircons that record prograde metamorphism thus faces serious challenges. A key finding, then, is that zircon age dates, especially bulk mineral TIMS ages, are unlikely to record the times of peak metamorphism, but should rather systematically underestimate the ages of peak UHP metamorphism.
 
 

Atom-by-atom mapping of minerals

On page 852 of this issue, Steve Parman presents an application of atom probe tomography (APT) to a natural sample of isoferroplatinum. As noted by Parman, APT is more commonly used in the materials sciences, but as demonstrated by this work, can also be applied to more compositionally complex natural crystals. The APT provides an atom-by-atom mapping providing both compositional, isotopic and structural information. It is not clear that APT can yet be applied to analyze covalently bonded silicates, but in the mean time, oxides, metals, and sulfides provide potentially useful targets, and if the silicate problem is solved, APT may serve to revolutionize the way minerals are analyzed and understood. 

 

Notable Papers for February-March 2015

 

 

Highlights and Breakthroughs -- Sulfur Solubility

On page 341 of this issue, Nowak provides an overview of experimental work by Huang and Keppler (page 257 of the January 2015 issue) who examine sulfur solubility in model rhyolite melts. They find that under reducing conditions, total CaO content has no effect on the partitioning of S between fluid and co-existing melts and S partitions strongly into a co-existing fluid phase. In contrast, under oxidizing conditions (NNO +0.5 or higher), where anhydrite is stable, increasing CaO contents cause a net decrease in S that is partitioned into a coexisting fluid, largely because with increased CaO, a greater degree of S is locked up in crystalline phase (anhydrite in this case). This result, and earlier work on the kinetics of anhydrite decomposition, imply that oxidizing conditions may inhibit the release of S into the atmosphere on the short time scales of volcanic eruptions.

 

Fluids Released from Subducting Slabs

On page 352 of this issue, Frezzotti and Ferrando review fluid inclusion compositions from high pressure (HP) and ultra-high pressure (UHP) lithologies. They suggest that the solutes in HP fluid inclusions are dominated by chloride salts, alkalis, and Si and Al, similar to species that are found in crustal fluids. At UHP conditions, however, solutes are dominated by a wide range of aluminosilicate components as well as sulfate and carbonate species, which appear to record the increasing solubility of mineral components into fluid phases. Of particular interest are the high amounts of carbonate at sub-arc pressures, which indicates that the transport of C in subduction zones may be as much affected by the transport of aqueous fluids as by other phases.

 

Shapes of Things (Plagioclase Microlites and Melt Inclusions)

On page 385 of this issue, Brugger and Hammer explain variations in plagioclase microlite morphologies, and how such variations might be related to decompression rates and the formation of twin-plane boundaries. Their experiments show that twin planes in plagioclase likely form at the early stages of crystal growth as a result of defects in the crystal structure. Such defects, and twinning itself, is much more prevalent at high effective undercooling. Of key importance is that these high-energy twin surfaces can inhibit new growth at or across twin boundaries. These findings suggest that melt inclusions, swallowtails, and hopper structures might not result from diffusion-controlled growth, but rather due to preferential crystal growth on lower-energy surfaces away from twin planes.

 

A Mystery of Mantle Sources

On page 396 of this issue, Cortes et al. examine the mantle source components for two phases of both closely and distantly spaced volcanoes in the Lunar Crater Volcanic Field of central Nevada. They examine four volcanoes that formed within two time windows; in both cases, the magmas appear to be generated at very similar depths and to record very similar transport and cooling histories. In the earlier window, however, both volcanoes are just hundreds of meters apart and yet tap a slightly more heterogeneous source compared to the two later volcanoes, which yield lava flows with much more homogenous compositions—despite being separated by several kilometers, and tens of thousands of years for their eruptions.

 

Water in Magmas

On page 466 of this issue, Le Losq et al. show that the structural volume of H depends upon the radius of alkali cations dissolved in a hydrous silicate melt. The total amount of water dissolved in their experimental glasses has no apparent effect on H volume. However, H volume is smaller when the melts dissolve alkalis with larger ionic radii. This probably participates in determining larger partial molar volume and higher solubility of water in Na- compared to K-bearing silicate melts. These results imply that both water solubility and water-controlled magma buoyancy are intrinsically affected by magma composition.

 

Peridotite Solves The Climate Change Problem

On page 474 of this issue, Peuble et al. examine the role of crystal-preferred orientations (CPO) with regard to olivine dissolution and carbonate precipitation. In natural peridotites affected by asthenospheric flow, olivines are oriented so that (010) planes are mostly horizontal (b-axis vertical), and (100) planes are mostly vertical (a-axis horizontal). In this experimental study, the authors find that upon injection of CO2-bearing fluids into such an anisotropic regime, olivine dissolves in the (010) direction, and carbonate precipitation is controlled by fluid flow along fractures in the 10 direction; with precipitation of carbonate, the latter event effectively blocks further fluid transport, limiting the re-emission of CO2.  If their experimental trials can be scaled upward, such anisotropy may provide a leak-resistant means for storing CO2.

 

Making Dolomite Fast

On page 483 of this issue, Zhang et al. investigate a new means by which disordered dolomite may be precipitated. Their experiments indicate that dolomite may precipitate directly from Ca-Mg-bearing solutions through the catalytic effect of extracellular polymeric substances (EPS), formed in association with anaerobic, fermenting bacteria. They posit that hydrogen bonds form between H and OH in EPS layers and O in CO32– at carbonate surfaces. The adsorbed EPS weakens bonding in the hydration shell about surface Mg cations, leaving Mg available to bond with carbonate. Their work provides the first demonstration that anaerobic bacteria may play a role in the direct precipitation of dolomite.

Notable Papers for January 2015

 

 

Editorial

On pages 1-2 of this issue, we present an editorial on the future of mineralogy and the American Mineralogist: Journal of Earth and Planetary Materials

 

Highlights and Breakthroughs

Defining Minerals & Mineralogy

On page 3 of this issue Peter Heaney provides some context of Caraballo et al. (page 14), whose Outlook paper examines the troublesome issues of classification of nano-materials. These materials may fall outside the boundaries of generally accepted mineral-defining characteristics, and yet be fundamental to naturally occurring and environmentally important systems. Heaneys overview provides food for thought regarding the importance of classification generally and in mineralogy especially, the latter topic of which is coincidentally covered from an historical standpoint in our Editorial.

 

Mineral Evolution

On page 4 of this issue, Dwight Bradley provides an overview of a series of publications by Robert Hazen and co-workers, including Hazen et al. (2011, 2012, 2013b) and Grew and Hazen (2014). Bradley suggests that an important step in mineral-evolution studies will involve the isotopic history of long-extant mineral species, such as zircon and calcite, whose isotopic ratios possibly track supercontinent cycles/subduction flux rates, but from different perspectives: continental growth and subduction flux on the one hand (zircon), and continental weathering and plate generation flux on the other (calcite).

 

Invited Centennial Article

Emplacement of High-Pressure Blocks in Subduction Zones

On page 6 of this issue Gary Ernst presents the inaugural paper in our Invited Centennial Article Series, which celebrates American Mineralogist's 100th volume. In this paper, Ernst presents a model for the origin of high-pressure (HP) meta-basaltic blocks in the Franciscan Complex. The HP blocks contain rinds of slightly younger actinolite that appear to record equilibration with harzburgite. Ernst suggests that such rinds record the means by which such blocks are transported towards the surface, i.e., by being carried upwards in subduction-channel muddy mlanges, or in buoyant serpentinized peridotite diapirs. Ernst also finds that some portions of such rock packages are exposed at the surface and eroded, so as to be later deposited back into the trench as olistostromes.

Fate of Subducted Carbonates

On page 35 of this issue, Foustoukos and Mysen report on the structure of melts in the systems MgO-H2O-CO2 and CaO-H2O-CO2. They hypothesize that carbonates subducted as sediment and serpentinized peridotite may not reach much greater than sub-arc depths. Instead, carbonates may completely partially melt prior to the completion of slab-dehydration. In such a case, subducted carbonates would not contribute C to the deep mantle and would instead contribute a CO2 flux into the crust and arc volcanoes, even in relatively cold subduction conditions.

 

Late Volcanic Degassing of F and Cl

On page 47 of this issue, Dalou et al. examine the structure of F- and Cl-bearing melts in simple alkali-silicate systems (Na2O-Al2O3-SiO2-H2O). Their work shows that F and Cl in silicate melts may be sequestered in the form of SiF and NaCl complexes, and the solubility of such complexes may increase with decreasing pressure. This result can explain the relative late degassing of F and Cl from volcanic systems, relative to degassing of CO2 and H2O.

 

Bond Valence Models Just Got Better

On page 148 of this issue Wander et al. improve the Bond Valence Model (BVM) for the prediction of local chemical structure. The BVM defines the valence of a bond as a function of bond length and two additional empirical parameters, and then uses Paulings second rule to solve for atomic valence. Current BVMs consider only cation-anion interactions, but Wander et al. show that the model can be improved by including anion-anion interactions (oxygen-oxygen in their case); they suggest that cation-cation interactions, while not useful in the system H-Al-Si-O investigated by them, might be useful for other systems. The hope is that a more accurate BVM may prove a useful tool to augment molecular dynamics simulations.

 

Deciphering Compression Mechanisms in Albite Liquids

On page 326 of this issue, Gaudio et al. examine the compressibility of albite (NaAlSi3O8)-composition glasses using a novel in situ high-pressure solid-state NMR technique.  Pressure-induced changes to the 23Na NMR spectra indicate that with increasing pressures, decreases in volume are accommodated almost entirely by shortening of Na-O bonds, rather than changes in coordination number for any of Na, Al, or Si.  The volume of the Na-O coordination spheres decreases by nearly 7% between 1 atm and 2 GPa. As Gaudio et al. note, such understanding has important implications for models of viscosity, elasticity and trace element partitioning in silicate melts. For example, the shortening of Na-O bond with increasing pressure may increase the effective field strength of Na, which would then stabilize high-coordinated Al, resulting in a reduction in viscosity of the melt with increasing pressure.

Notable Papers for November-December 2014

Two Highlights and Breakthroughs

Recording Water on the Martian Surface

This issue features two Highlights and Breakthroughs articles. On page 2161 of this issue, Brad Jolliff highlights the work of McCubbin et al. (page 1347 of the July issue). Their careful analysis of phosphate minerals in the Shergotty (martian) meteorite shows that the occurrence of anhydrous merrillite (vs. hydrous whitlockite) is not necessarily an indicator of anhydrous crystallization conditions. Instead, the competing phase assemblages of apatite + merrillite vs. apatite + whitlockite may reflect the role of temperature in controlling H activity in these phosphates.

 

Assessing oxidation states on Mars

On page 2163 of this issue John Bridges provides a review of Treiman et al. (2014; page 2234 of this issue), who examine a terrestrial analog of martian smectities observed using the ChemMin XRD instrument aboard the Mars Science Laboratory. Their spectroscopic work shows that their analog mineral, Griffithite, contains significant Fe3+, which they hypothesize to be secondary, forming during diagenesis. They find that very near IR reflectance (VNIR) spectra vary with Fe3+ content, indicating that VNIR may be used to remotely assess oxidation states on planetary surfaces.

 

Abductive Reasoning

On page 2165 of this issue, Bob Hazen considers abductive reasoning in the Earth Sciences, and the newly emerging role of large databases. Abduction is a logical form posited by Charles Pierce, which effectively means guessing. For example, if you come home to find your favorite biking shorts in shreds, your dog has a guilty look, and bits of Lycra are scattered about his bedding, you may abduce that your dog ate your biking shorts. Or course, they may have been shredded by a strange dog that hopped the fence and entered through the doggy door, or by an ex-cycling partner who stole into your house with a pair of scissors. But your black lab is a more likely suspect. Hazen argues that newly-developed large data sets in the Earth and Biological sciences can lead to a new era of data-driven, abduction-oriented discovery. The new data sets are expected to be a rich source of useful working hypotheses in the coming decades. But Hazen warns that to yield their full potential, such databases require concerted efforts on the parts of publishers, authors and data compilers, to ensure that data are properly systematized and archived, and that such archived data are of high quality.

Phenocryst Residence Times

On page 2211 of this issue, Grant et al. experimentally investigate the growth of phlogopite reaction rims on olivine, for phonolitic whole rock compositions. Their work shows that the growth rates of phlogopite reaction rims are limited by diffusion along phlogopite-phlogopite grain boundaries of the growing rim, and that increased water contents greatly increased rim growth rates. This is due to the lowering of diffusion rates by the presence of "atomically bound" fluid at grain boundaries. Their results also appear to indicate presence of pore cavities, which might not have formed during de-pressurization; rather, pores with negative phlogopite geometries may indicate the presence of "free" fluid at grain boundaries that formed at peak experimental (and bulk fluid-undersaturated) P-T conditions. If this latter observation is valid, then disequilibrium fluids may have an important effect on our interpretation of various diffusion-mediated processes in magmatic systems

 

The Origin of Group II Kimberlites

On page 2292 of this issue Sokol et al. present results from partial melting studies that may shed light on the origin of Group II (phlogopite-bearing) kimberlites. Their experiments are performed on a synthetic kimberlite, representing a mean of published Group II compositions. If such published compositions approximate a magmatic composition, then the experiments by Sokol et al. delimit several intensive parameters by which such liquids were generated. They obtain liquids saturated with olivine + orthopyroxene + garnet, which are compositionally similar to minerals retrieved from peridotite inclusions. None of their liquids were saturated with clinopyroxene at any conditions examined. Their data allow that kimberlite-like liquids can be produced by partial melting of a carbonated garnet harzburgite source at 6.3 to 7.5 GPa and 1500 oC, with water contents restricted to <5 wt%. The water may have come from reaction of resident phlogopite with K-Ca-rich carbonated melts derived from a deeper mantle source.

Hydrating the lower mantle

On page 2374 of this issue, Zhang et al. examine the pressure-density relationships (through the Hugoniot equation) for various  decompositional equilibria for antigorite. They find that the high pressure phase H, MgSiO4H2, may be a stable decomposition product of antigorite in the pressure range 40-70 GPa. If valid, these results imply that antigorite might not dehydrate until it reaches lower mantle depths, in which case phase H may be an important water delivery system from the near surface into the lower mantle. The authors also suggest that antigorite and possibly phase H (if it is quenchable to low pressure) may survive shock impact pressure and so provide a means of hydrating planetary interiors during planetary accretion at near-solar distances.

Al in Earths Core?

On page 2433 of this issue, Bindi et al. report on a new mineral named steinhardtite, which represents the first natural occurrence of a body centered cubic (bcc) polymorph of an Al-bearing compound, from the Khatyrka meteorite. The structure exhibits solid solution with Ni and Fe with a formula of Al0.38Ni0.32Fe0.30. A key result of this work is that the solution of Fe and Ni into the structure reduces the pressure at which Al undergoes an hcp to bcc transition, the latter of which should be stable in Earth's core. These results thus indicate that within the Earths core, Al might be a plausible candidate as a light alloying element.

Notable Papers for October 2014

 

Three Highlights and Breakthroughs

Paulings Rules

This issue features three Highlights and Breakthroughs articles. On page 1817 of this issue, Bob Downs provides a review of Gibbs et al. (2014) re-analysis of Pauling's rules, and his use of a saddle point in the electron density between two bonded atoms so as to mark a non-spherical radii. This new view of the "shape" of an atom provides a challenge to our use of certain Pauling's Rules, where, for example, a spherical atom is assumed so as to determine coordination number from radius ratios. The model presented by Gibbs et al. also provides a new perspective on the meaning of bond strength.

 

New uses for vermiculite

Then on page 1818 of this issue, Marcos discusses new work by Kaur et al. (2014; page 2018 of this issue), whose studies of gamma-irradiated vermiculite show that this phase has high radiation shielding properties and chemical stability despite radiation bombardment; these properties open the door for the development of new types of opto-electronic devices, and a possible new radiation dosimeter.

 

Caveats for Ti-in-quartz thermometry

Finally, on page 1820 of this issue, John Hughes reviews work by Ashley et al. (2014; page 2025 of this issue), who use single-crystal XRD and atomistic simulations to test two modes by which Ti may be redistributed in recrystallized quartz. They conclude that strain-driven redistribution may be important, with localized thermodynamic equilibration with the intergranular medium buffering Ti solubility in quartz. Ti-in-quartz thermometry may thus be compromised when quartz is dynamically recrystallized.

 

Crystallization ages from actinide-rich phases

On page 1985 of this issue, Cottle presents new LA-MC-ICPMS data from thorite and uranothorite [(Th,U)SiO4] crystals that range in age from 13 to 500 Ma. These single-crystal age determinations are concordant in the U-Th-Pb system, and so appear to reveal accurate dates of crystallization, with little loss of Pb despite the potential for radiation damage. The method can provide spatial resolution as fine as 5 micrometers, and thus age-dating of actinide-rich minerals should provide a valuable age dating tool that complements single-crystal zircon chronometry.

 

Negatively buoyant sediments in the deep mantle?

On page 2035 of this issue, Andrault et al. show the results of experiments on Al-bearing SiO2, at high pressures and temperatures. Their results show that in SiO2 phases with 4-6 wt% Al2O3 there is a ~2-3% density increase for the transition from a CaCl2-structured phase, to the a-PbO2 structure (seifertite) at 113-119 GPa and 300 to 2000 K. Because of the small amount of Si in a subducted MORB, the net effect of this phase transition should be negligible. But since overlying sediments might have considerable SiO2, this CaCl2-seifertite phase transition may have a significant impact on the negative buoyancy of subducted sediments, and perhaps facilitate the descent of the sedimentary material to the lowermost part of the mantle.

 

Decompressed glasses are OK

On page 2142 of this issue, Malfait et al. show that measured compressibilities of glasses can be used to accurately calculate in-situ, high pressure-temperature densities and that any structural changes during decompression must be small. Therefore, ambient-pressure spectroscopic measurements on quenched glasses reflect those of the high-pressure structures. This work gives reassurance that various published work on high-P glasses is relevant to melt structure issues of most interest to geologists and geophysicists.

More precise geobarometery

On page 2146 of this issue, Angel et al. re-examine a key issue of geobarometers based on residual stresses that can be observed between an inclusion and a host phase. In their new approach, they do not assume that elastic properties of the inclusion and host are linear with respect to changes in P and T. Instead, they make use of isomekes, paths in P and T along which the fractional volume changes of host and inclusion are identical. Their approach is independent of the type of equation of state used for any given system and provides a more detailed physical basis to the barometry problem, which in turn allows for a more accurate determination of entrapment pressures and temperatures from residual stress measurements.

 

Notable Papers for August-September 2014

A New Method to Investigate Deep Earth Processes
On page 1521 of this issue, Wu and Buseck use carbon nanocontainers, examined using TEM, to study Earth materials at high pressures and high spatial resolution. The advantage of their approach is the possibility of observing the progress of high P-T reactions at nearly the atomic scale. This technique may be especially important in the investigation of unquenchable phases and reactions at a spatial resolution that is not possible with other techniques. A preliminary application of the technique allowed the authors to discover C concentrations in stacking faults in TiO2 at 8 GPa, which provides a new means for storing C in the deep mantle.

 

Making Polycrystalline Diamonds

On page 1537 of this issue, Mikhail et al. investigate N contents and aggregation states in diamondites (polycrystalline diamonds) using IR spectra; the samples are inferred to derive from various parts of Southern Africa. Nitrogen contents vs. aggregation states of diamond provides constraints on the temperature history and mantle residence time. Their work indicates that residence times among different diamondite samples vary significantly. Another possible interpretation is that diamondites have similar residence times but form over a large depth interval, an hypothesis that is rejected since diamondites are expected to form at low melt fractions (which are thought unlikely to exist over a large depth range at any given time), and the composition of their syngenetic garnet inter-growths do not show compositional evidence to support variable depths of formation. The variable residence time interpretation requires multiple growth events, which is consistent with what is observed for the formation of monocrystalline diamonds. This work contributes to the question of why some kimberlites yield diamondites and monocrystalline diamonds, but some only yield monocrystalline diamonds. This question is still open, and requires further work to decipher if the diamondites share a genetic link with their monocrystalline gem-bearing counterparts, or not. But for now these data show for certain that diamondites are not related to coated diamonds and likely have little to do with proto-kimberlitic fluids, as previously thought. 

 

Storing Carbon as Partial Melt in the Deep Mantle

On page 1544 issue Thomson et al. conduct high P partial melting experiments in the system MgSiO3-MgCO3. They find that low-degree partial melts of subducted carbonates may be widespread in the convective lower mantle. They hypothesize that, during subduction, partial melting of carbonates is initiated near the upper/lower mantle boundary and that this may explain so-called superdeep diamonds, which are thought to derive from this depth interval. Their results also indicate that the slope of melting curves in this system (dT/dP) approach zero, or perhaps even become negative, at high pressures such that if S is positive as expected, carbonate-rich partial melts may be equally or more dense than bulk equilibrium solids. Such a characteristic would allow carbonate to be trapped in the lower mantle in the liquid state, providing the possibility of a carbon-rich reservoir in the deep mantle.  

 

The effects of fO2 on Carbon Mobility

On page 1604 of this issue—in our Fluids in the Crust Special Section—Lazar et al. investigate the effects of fO2 on the dissolution and mobility of C in silicate systems. They find that at fO2 values of about 2 log units below QFM or lower, C mobility may be at a maximum as calcite dissolves and C migrates into fluids, mostly in the form of methane. The authors indicate that such reducing conditions may be found in a number of geologic environments—but may be especially important during serpentinization, where fO2 may be as low as 7 log units below QFM. The authors suggest that C mobility in subduction zones may be slightly limited by graphite saturation, but may even then still be quite mobile, with or without graphite. Their work also implies that calcite reduction may also be important in abiotic methanogenesis, a process that may be ubiquitous in subduction or astrobiological environments.

 

The Reason for HFSE Depletions in Arc Magmas

On page 1616 of this issue Louvel et al. conduct high P-T experiments to examine the partitioning of Zr between aqueous fluids and hydrous, F-bearing haplogranitic melts, using in situ synchrotron X-ray fluorescence. Their experiments show that Zr is not quantitatively excluded from fluids and that fluid-melt partition coefficients are >0.1, with or without F in the melt. Their work, and related studies, indicates that the HFSE (high field strength element) depletions characteristic of arc magmas might not be controlled by partial melting within or above the slab, but rather that fluid-rock interactions near the slab interface sequester HFSE, as fluids migrate from the slab into the overlying mantle wedge. Upon such reaction, hydrated peridotites just above the slab/wedge interface would then precipitate HFSE-bearing phases (rutile, clinopyroxene, garnet), which are presumed to be carried into the transition zone.

Notable Articles for July 2014

Highlights and Breakthroughs

We have three Highlights and Breakthroughs articles in this issue. On page 1193, Mysen provides an overview of experiments by Shinozaki et al. (p. 1265 of this issue), which show that Mg/Si ratios of the mantle may be reduced if metasomatized by high H2+H2O fluids, compared to metasomatism by H2O alone. The relevant reactions create potentially novel Si-hydride species, increase the oxygen fugacity of the mantle, and may affect the SiO2 content of mantle partial melts. On page 1195 of this issue, Hatert reviews a recent paper in our April issue by Chukanov et al. (2014), who describe a new phosphate; as explained by Hatert, this mineral is one of an inexhaustible store of new structures used by materials scientists, and may provide new thermometers applicable to pegmatitic systems. Finally, on page 1197 of this issue, Topper provides a review of Proyer et al. (p. 1429 of this issue) who establish one of the first petrogenetic grids for subduction related rocks based on Ti- and Zr-bearing accessory minerals. These thermodynamic constraints allow the authors to reconstruct a retrograde P-T path in a continental collisional zone at UHP conditions.

Providing a habitable chemistry on Mars
On page 1206 of this issue, Tu et al. examine the dissolution rates of amorphous Al- and Fe-phosphates. These authors show that both phases dissolve rapidly under acidic conditions and that amorphous Al-phosphates dissolve approximately three orders of magnitude faster than their Fe-bearing counterparts. This work is important because P is an essential element for life as we understand it. These experiments thus show that amorphous phosphates could have provided P to near surface fluids, so as to support biological processes on Mars.

A Mineralogical Mechanism To Uplift The Colorado Plateau

On page 1277 of this issue, Schulze et al. report the discovery of guyanaite (CrOOH) in a pyroxenite xenolith from the Navajo Volcanic Field. The pyroxenites are similar to jadeite blocks from convergent margin serpentinites, and thus the authors interpret these xenoliths as remnants of the subducted Farallon plate. Here, guyanite is thought to result from Na-alteration of a serpentinite during subduction and evolved on a prograde metamorphic path involving dehydration. This work thus appears to confirm that oxy-hydroxide minerals (e.g., AlOOH, FeOOH, etc.) can transport water into the deep mantle, and lends support to the idea that breakdown reactions may control mantle rehydration, which in turn may influence volcanic activity and in this case, uplift of the Colorado Plateau.

UHP Conditions at 1.8 Ga

On page 1315 of this issue, Glassley et al. report a series of mineralogical clues that indicate UHP metamorphism in 1.8 Ga rocks from West Greenland. These are the oldest UHP rocks reported to date, and nearly double the age for outcrops of such terranes. Among the observations are quartz needles in fayalite, which they interpret as silica that has been exsolved from b-spinel that incorporated excess silica. Reaction relationships reveal a remarkably low T of equilibration at high P: 975 C at 7 GPa. If the P-T conditions are real, then a key unresolved issue relates to whether the very cold conditions at such depths are unusual, or if such conditions are repeatable, and can be obtained when subduction is sufficiently rapid.

Wet Meteorites

On page 1347 of this issue, McCubbin et al. verify the presence of merrillite with OH-rich apatite in the martian Shergotty meteorite. Unlike whitlockite, merrillite does not contain water as an essential structural constituent. The lack of whitlockite in meteorite samples has thus been interpreted to mean that such samples were dry at the time of formation. McCubbin et al.s discovery indicates that the absence of whitlockite is not a reliable indicator of anhydrous conditions, and that the absence of whitlockite and the presence of merrillite might instead indicate not so much dry conditions but rather temperatures of equilibration in excess of 1050 C.

Cleansing Contaminated Waters with Minerals

On page 1355 of this issue Mills et al. investigate Sb substitution mechanisms in segnitite, using a sample from the Black Pine mine, Montana, U.S.A. Antimony is a toxic heavy metal that is often found associated with weathering products of mine waste and tailings. Clearly, a means to remove Sb from mine drainage sites is an urgent need. Mills et al. show that at very low pH (>2) the mineral segnitite, which strongly partitions Sb, could be extremely important for the removal of not just Sb, but also Pb, As, and Fe from within the supergene zone.

How to Grow Big Crystals Fast

On page 1383 of this issue, Maneta and Baker conduct experiments to explore the effect of Li as a catalyst for the growth of quartz and feldspar from hydrous silicate melts. Their study shows that with the addition of 3700 ppm Li, graphic and granophyric intergrowths of quartz and feldspar occur at about half the supercooling (~100 C) required in the absence of Li (~200 C or more).  Moreover, growth rates for feldspar (13 cm/yr) are approximately an order of magnitude greater than in the Li-free system. The crystallization interval is also greatly reduced, implying that crystal growth rates must be rapid for all phases, for a given isobaric loss of thermal energy. Lithium is thus perhaps the most efficient of possible fluxing agents.

Notable Articles for May-June 2014

Highlights and Breakthroughs

We have two Highlights and Breakthroughs articles in this issue. On page 877 of this issue Lee provides an overview of a paper published by Mao et al. on the spin state of Fe in silicate glasses under compression. Their work shows that Fe, whether 3+ or 2+, retains a high spin state at pressures up to 120 GPa. Thus, to the extent that glasses might represent an appropriate analogue for liquids, spin transitions for Fe within the liquid phase are unlikely to explain their macroscopic properties or partitioning behavior in the deep Earth. Then on page 879 of this issue, Poli provides perspective on dolomite solid solutions, as investigated by Li et al., who describe oscillatory zoning in dolomite that describes in detail a prograde metamorphic path within a subduction zone. As noted by Poli, oscillatory zoning may be driven by changes in fluid compositions during the prograde decomposition of amphibole and lawsonite, and that the trace element profiles of carbonates may provide a yet-to-be tapped treasure trove of information on subduction-related metamorphism.

Apatite: the (nearly) ideal mineral

On page 890 of this issue, Hovis et al. provide unit cell and solution calorimetric measurements that show that mixing along the binary fluorapatite–hydroxylapatite is nearly ideal, with no excess volume of mixing, and only minimal negative excess heats of mixing. Their work just shows that there is no thermodynamic barrier to a complete solid solution between these end-members, which means that the F/OH ratios of natural apatite compositions will very directly reflect the F/OH ratios of the fluids or magmas from which they precipitate.

Plagioclase Crystallization Kinetics

On page 898 of this issue, Iezzi et al. conduct cooling rate experiments on an andesite bulk composition. Their work shows that at higher cooling rates (>12.5 C/min) plagioclase grains tend to be enriched in An content relative to equilibrium values. Perhaps more interesting is that at intermediate cooling rates (3 C/min) two populations of plagioclase are evident, one that is at disequilibrium and enriched in An, and another population that approaches an An-poor composition that mimics equilibrium conditions. Apparently, such intermediate cooling rates are fast enough to force disequilibrium growth of high-An plagioclase, but still low enough to allow low An plagioclase to nucleate in the residual melt.  This result may directly apply to lava flows that have thicknesses of a few centimeters to a few meters, where cooling rates will be within a range that may allow the nucleation and growth of both near-equilibrium and disequilibrium plagioclase compositions.

Degassing Paths from Melt Inclusions: A Caveat

On page 976 of this issue, Esposito et al. examine associated melt inclusions, where petrographic evidence indicates simultaneous trapping. Such melt inclusions tend to be very consistent in terms of major oxide and H2O contents, but they contain highly variable amounts of CO2. Provided that these MI were indeed trapped at the same time, these results indicate that H2O-CO2 trends, where H2O contents are nearly invariant, might not represent a magma degassing path. Instead, such paths may reflect variations in CO2 during trapping, or post-crystallization loss of CO2 from the MI. 

Evolution of Be-bearing minerals

On page 999 of this issue, Grew and Hazen examine the possible evolution of terrestrial Be-bearing minerals. Their work indicates a near steady increase in diversity of Be minerals, interrupted in places by plateaus of negligible diversity increase, lasting 100 Ma or longer. However, the authors note that such plateaus may depend as much on chance rather than causative conditions, since nearly half of all Be minerals are found in just a few locations of unique chemistry.

Keystone minerals during Serpentinization

On page 1035 of this issue, Miyoshi et al. show that orthopyroxene is something like a keystone mineralogical species in serpentinization reactions. The breakdown of orthopyroxene provides excess silica that can then react with brucite to form magnetite. The formation of magnetite further contributes to the electro-magnetic characteristics of the bulk rock, and moreover release H that may be crucial for the development of microbial communities on the ocean floor.

Magma ascent rates

On page 1052 of this issue, Fiege et al. conduct experiments to examine bubble growth and bubble number densities (BND) in andesitic melts. Their results show that, for a given pressure drop, bubbles nucleate more readily along single step or multi-step decompression paths compared to a continuous decompression path. They also find that melt composition, rather than temperature, plays an important role for bubble formation. For example, at a given decompression rate, andesitic melts yield much higher BNDs compared to rhyolitic melts. Such compositional dependencies are not described by existing models of bubble nucleation, and so may cause overestimates of magma ascent rates by as much as an order of magnitude.

Carbonatite and Kimberlite Genesis

On page 1119 of this issue, Keshav and Gudfinnsson conduct partial melting experiments in the system CaO + MgO + Al2O3 + SiO2 + CO2. They find that at high pressures of 8-12 GPa, heating along an isobaric path produces a smooth continuum in liquid compositions that are in equilibrium with four phase assemblage of forsterite + orthopyroxene + clinopyroxene + garnet. Along an isobar, at low temperatures, the liquids are model carbonatites, and at high temperatures, liquids are model kimberlites. At the investigated pressure-temperature conditions, Keshav and Gudfinnsson also find that in terms of oxide ratios, individual isopleths are very close to each other than is the case at lower pressures of 3-8 GPa (Gudfinnsson and Presnall 2005). On this basis, the authors suggest that further increase in pressure will not have much effect on liquid compositions, and it might be that most primary kimberlites are just limited to upper mantle depths in Earth. In this manner, diamonds that are interpreted to have come from depths approaching those of the Transition Zone, or even deeper, might have come via mantle flow to shallower depths where kimberlites are primarily generated. From such regions in the mantle, later generation kimberlites bring diamonds to the Earths surface.

Nuggets of Truth
On
page 1171 of this issue Brugger et al. examine the origins of microscopic gold. Also known as mustard gold, it is a common weathering product of Au-Te ores, and it can account for a large portion of the gold in economic deposits. The authors present compelling laboratory evidence that Au-Ag-tellurides can be transformed to form a mixture of microporous gold and tellurite or Te-associated iron oxides, which bear a striking resemblance to the textures seen in the natural deposits.

Notable Articles for April 2014

Highlights and Breakthroughs

We have two Highlights and Breakthroughs articles in this issue. On page 561, Lofgren provides perspective on a recent paper by Elardo et al. who find zoned pyroxenes in lunar basalts. Pyroxene grains exhibit oscillatory zoning that cannot be the result of interface-mediated kinetic processes, and so must reflect magma wide processes where pyroxenes are exposed to different environment as they grow. Then on page 562, Pasteris provides perspective on a recent paper by Rollin-Martinet et al., who apply thermodynamic laws to understand how bone material (apatite) matures in biological systems. This new work shows that the chemical reactions of maturation can be explained as thermodynamically driven and as the reason that old bone apatite needs to be replaced by new apatite to retain its biological functionality. The next step in such work is to evaluate the thermodynamic effects of carbonate constituents in bone material.

Which cools quicker: calc-alkaline or tholeiitic basalts?

On page 564 of this issue, Hofmeister et al. measure thermal diffusivity on a range of silicate liquid compositions. Their work shows that Ca and Fe both have a significant effect on liquid thermal diffusivities. The result is that at lower temperatures, Ca is the key control, where calc-alkaline magmas should cool more quickly than tholeiitic basalts and I-type granitic liquids should cool more quickly than lower-Ca A- and S-type magmas. At higher temperatures, the effects of Fe on diffusivity may become more important, which may reverse the order.

Si in the cores of Mars and Mercury

On page 720 of this issue, Geballe and Jeanloz examine the phase transition of FeSi to a CsCl structure at 30 GPa; this phase transition is accompanied by a 5% increase in density relative to the low-pressure precursor, as Si increases from seven- to eightfold coordination. This increase in Si coordination has implications for the solubility of Si in the cores of Mars and Mercury. On Mars, for example, the CsCl phase of FeSi may be stable through the entire depth extent of the martian core. The implied increase in the effective radius of Si dissolved in metallic Fe suggests that Si may be even more soluble in the cores of these planets than previously thought.

Mantle dehydration and the isotopic composition of the oceans

On page 724 of this issue, Yang et al. examine H bonding strengths (through OH stretching frequencies) in ringwoodite and wadsleyite—two minerals that are key to understanding the storage of water in Earth's mantle. The low OH-stretching frequencies discovered for both these phases imply that H/D ratios may be strongly fractionated if either of the crystalline phases are in equilibrium with a fluid. If degassing of Earth's mantle was responsible for the formation of the oceans, then such fluid-mineral fractionation could explain the deuterium depletion of Earth's mantle relative to the oceans.

A new amphibole-amphibole metamorphic thermometer

On page 730 of this issue, Jenkins et al. perform experiments to examine the miscibility gap in amphiboles that fall along the tremolite-glaucophane join. As miscibility gaps are temperature sensitive, their work provides a new basis for estimating temperatures in medium- to high-grade metamorphic assemblages. This better-defined miscibility gap can also be used as a test of equilibrium. The authors show, for example, that in some reported cases, putative miscibility gaps are too wide to represent equilibrium, and so the amphiboles in question are more likely mechanical mixtures, rather than equilibrated dissolution pairs.

Boron nitride is a mineral

On page 764 of this issue, Dobrzhinetskaya et al. present the first description of cubic BN, which they term qinsongite. The bonding in BN is much like that in diamond, and synthetic BN, with similar hardness, is known as the industrial material Borazon.  These authors have found a natural sample of BN in Cr-rich oceanic crustal rocks of Tibet, which formed during the closing of the Tethys.  This natural BN was apparently formed as the crust was subducted to >300 km depths.

A caveat for cooling rates inferred from Cr-spinel ordering

On page 839 of this issue, Perinelli et al show that the Fe3+ contents of spinels can greatly impact rates of intracrystalline cation exchange. They examine intracrystalline partitioning of Al and Cr in two different suites of peridotite xenoliths. One suite records temperatures appropriate for ambient mantle conditions, while a second suite records much cooler conditions. They find that the inferred T contrasts between are unlikely to reflect cooling rate, but rather that the cooler recorded temperatures are a result of higher Fe3+ contents, which facilitate more rapid cation ordering. 

Amphiboles as a key to unlocking liquid compositions

On page 852 of this issue, Giesting and Filiberto exploit experimental results on amphibole-volatile equilibria to provide a means for estimating OH-, Fe3+ in amphiboles, and if Cl is measured, to use these estimates and partitioning relationships to determine H2O contents for coexisting melts.  As these authors point out, amphibole complexity provides igneous petrologists with great challenges, but also great potential, especially for assessing difficult-to-measure quantities such as the pre-eruptive water contents of co-existing liquids.

Cr in the deep mantle?

On page 866 of this issue, Bindi et al. describe the first example of the mineral perovskite (MgSiO3) to show appreciable levels of Cr in solid solution, at the level of 7 wt%. Chromium was found to substitute for both Mg and Si in equal proportion, by the scheme Mg2+ + Si4+ = 2Cr3+, lengthening the "Si-O" and shortening the Mg-O bond lengths, and increasing the tilting of the octahedra with respect to pure MgSiO3. Perovksite may have the ability to store appreciable Cr in the lower mantle.

Notable Articles for February-March 2014

Trading CO2 for Methane

On page 253 of this issue, Jung provides a perspective on a new study on the trick of injecting CO2 into methane-bearing sediments, to both store CO2 and to release CH4.  He argues that work by Hyodo et al., published last month in the American Mineralogist, provides an important first step for understanding the mechanical stability of methane-bearing sediments following CH4-CO2 exchange. A key problem that must be understood is the nature of volume changes, which can be as high as nearly 400%, if the un-exchanged sediments contain up to 50% methane. Such volume exchanges may allow fracturing, especially in low-porosity systems.

Weathering the Martian Surface

On page 283 of this issue, Kong et al. consider the formation conditions of kieserite, a hydrated sulfate that is widespread on the martian surface. Their work indicates that kieserite is a weathering product, rather than a primary aqueous precipitate. Rather than providing evidence for brines at the martian surface, kieserite instead forms as a dehydration product of hexahydrate. Weathering occurs over a span of 6 months—the length of a martian summer—as wind and sunlight drive dehydration. The resulting kieserite then appears to survive the colder, wetter martian winter.

Stirring magmas beneath the lunar crust

On page 355 of this issue, Elardo and Shearer discuss the unusual occurrence of oscillatory-zoned minerals contained within a Moon-derived meteorite. Although common among terrestrial volcanic rocks, this is one of only a handful of reports of oscillatory-zoned lunar minerals. The authors hypothesize that the implied magmatic convective system that created such zoning was a result of the storage of magma within a relatively cold crust, even at 3 Ga. Their work thus implies that the lunar crust was loosing heat rapidly, or at least rapidly enough to maintain a strong thermal contrast between crust and stored magmas, to yield a vigorously convective system. 

Preserving high pressures

On page 433 of this issue, Kouketsu et al. provide a novel method for estimating peak pressures in metamorphic systems. A key problem in the application of traditional compositionally based thermometers and barometers is that mineral compositions may be reset as rocks follow a retrograde metamorphic path en route to the surface. Kouketsu et al. get around this problem by examining shifts in peak positions in Raman spectra of quartz inclusions contained within garnet hosts. Elastic modeling shows that the magnitude of such peak shifts are a function of the pressures at which the quartz grains were originally included within the garnet, during garnet growth. Most importantly, this elastic information is retained even when surrounding matrix materials are thoroughly recrystallized, thus apparently preserving peak metamorphic pressure conditions.

Shocking insights into the graphite to diamond transition mechanism.

On page 531, Garvie et al. examine nano-sized grains of interstratified graphite and diamond from Gujba, an extraterrestrially shocked meteorite using HRTEM. This coexistence allows them to derive a topotactic mechanism for the transition, complete with the transition matrix. The findings provide new mechanistic insights into the interactions that control the transformation of graphite into diamond.

Chilling conclusions on pegmatite formation

On page 543, David London shows that it is the extent of liquidus undercooling that determines whether or not zoned pegmatites or granites form. When a liquid is undercooled, the driving force for crystallization may differ substantially for each phase, even for eutectic compositions. Pegmatite formation occurs via the sequential (rather than simultaneous) crystallization of feldspars and quartz from liquids that possess near-minimum or eutectic compositions. This occurs via subsolidus isothermal fractional crystallization, which generates the characteristic zonation. Consequently, for natural liquid compositions that contain at least tenths of a weight percent of CaO, feldspars predominate over quartz, plagioclase precedes K-feldspar, and the most calcic plagioclase and mafic minerals, with their higher actual liquidus temperatures, crystallize first.

A new facet of mantle diamond origins

On page 547, Zedgenizov et al. report the first occurrence of merwinite as an inclusion in diamond originating from the Sao Luiz alluvial deposits, Juina, Brazil. Merwinite cannot form under eclogitic or peridotitic mantle conditions, and its presence is evidence of a quite different Ca-rich, Mg-poor, and Si-poor mantle composition. It is suggested that subduction-derived calcium carbonatite melt reacts with host peridotite to form merwinite, and that its presence may be an indicator of Ca-carbonatite metasomatism in the deep mantle.

Notable Articles for January 2014

A Distinct Variety of Mineral in Bone

On page 1 of this issue, Hughes provides an overview of new work on the mineralogy of bone material, by Pasteris et al., which appears on page 16 of this issue. As noted by Hughes, this study shows that channels within the apatite structure are filled with water molecules, which help stabilize the channel structure even when as much as 80% of hydroxyl sites are depopulated and that in bone mineral, molecular water does not occur accidentally. Instead, water is an essential structural constituent in bone apatite, and thus bone comprises a definable variety of this mineral.

Immobilizing Co and As

On page 44 of this issue, Markl et al. investigate the conditions under which Ca-Mg-Co arsenates and carbonates are stable. Their work uncovers the pH conditions and Ca2+, Mg2+ activities that control the precipitation of various Co- and As-bearing phases, some of which can be used to immobilize Co and As near ore deposits. This work also explains the precipitation of various rare minerals near ore bodies, some of which may buffer Co and As concentrations in accompanying fluids.

Carbon in the Deep Mantle

One possible means of C storage in the deep mantle may involve magnesiosiderite (Mg0.35Fe0.65CO3), provided that such a phase is a stable part of a deep mantle assemblage. On page 84 of this issue, Liu et al. show that the low-spin form of magnesiosiderite is significantly more dense compared to end-member magnesite and high-spin magnesiosiderite. These results imply that at >50 GPa, low-spin ferromagnesite (which contains up to 20 mol% Fe) could be a stable form of carbonate at higher pressures and thus provide a significant storehouse for C in the deep mantle.

 

Constraining Light Alloying Element(s) in Earths Inner Core
Seismic studies have long shown that Earths core cannot consist only of crystalline Fe, as seismic velocities for both the inner and outer core are too low; hence, the core must contain one or more light alloying elements, and sulfur is a leading candidate for such. On
page 98 of this issue, Kamada et al. present new data that should help to advance our ability to for S in the inner core. They present an orientation-averaged value for Vp of Fe3S (a likely stoichiometry of Fe sulfide) at 24 < P < 85 GPa and 300 K, i.e. above the magnetic transition, and they calculate the dependence of Vp on density at pressures appropriate to the inner core using Birchs Law. They show that at the density of the inner core, both Fe3S and hcp-Fe yield a Vp that is too high relative to observed values, which means that either Brichs Law can be more temperature sensitive than now recognized, or that the inner core contains alloying elements/compounds that little effect the thermal sensitivity of Birchs Law.  

Mining Methane while Trapping CO2

Methane hydrates, which are dispersed amongst marine sediments, may provide an important source of energy in the coming decades. One proposed means of methane production involves CO2-CH4 exchange, as CO2 is injected into methane reservoirs. If feasible, this technology may not only spur energy production, but provide a means to simultaneously sequester CO2 in marine sediments. On page 178 of this issue, Hyodo et al present triaxial deformation experiments which indicate that sediment-bearing CO2-hydrates should be mechanically stable following CO2-CH4 exchange and CH4 extraction, which adds promise to this form of methane mining.

A Noble Cause
On
page 184 of this issue, Matsui et al. show that helium can be far from the inert gas it is assumed. Helium has previously been shown to stiffen silica glass, but Matsui et al. show that it may enter the interstices of the cristobalite framework to form stoichiometric SiO2-He structures in which it occupies well-defined crystallographic sites. The SiO2-He phases have molar volumes more than 20% larger than their SiO2 counterparts, and the presence of helium may modify both the positions of phase boundaries and symmetries of the structures. Helium may thus not always be the "ideal gas" for high-pressure experiments.

A Record of Prograde Subduction Metamorphism
On
page 206 of this issue, Li et al. investigate zoning profiles within eclogite-hosted dolomite, from the Tianshan area of northwest China. Thermodynamic modeling shows that Fe-Mg zoning patterns within dolomite record increases in T during prograde, subduction-driven metamorphism. These zoning profiles also show that Fe-rich magnesite, which occurs as inclusions in matrix dolomite and dolomite inclusions in garnet, grows at high-pressure metamorphic conditions, and thus that Fe-bearing magnesite is not an unambiguous indicator of ultra-high pressure metamorphism.

Notable Articles for November-December 2013

A Win for Interdisciplinary Collaborations

On page 1917 of this issue, Elsen et al. provide perspective on the recent paper by Jackson et al. (2013), which reveals the mineralogic and geologic aptitude of the Romans during the Axial age. Their study uncovered the Roman secrets for formulating some of the most long-lasting concrete yet discovered. Elsen et al. emphasize that our ability to unlock the secrets of ancient concrete formulas is dependent upon interdisciplinary analytical approaches utilized by the Jackson et al. (2013) group.

A Tektite Mystery

On page 1930 of this issue, Giuli et al. (2013) confirm earlier results from tektites and microtektites, that microtektites recovered from North America are significantly more oxidized than those recovered from near equatorial or southern hemisphere latitudes. Their current work shows that the observed range in oxidations states is unrelated to bulk composition but, at least in the case of the North American microtektites, is related to flight distance from the source crater. The reasons for the contrasts are still uncertain, but may be related to the thermal conditions of microtektite and tektite formation, and/or interaction of tektites with water-rich vapor plumes that are generated upon impact.

Evolution of Clays

On page 2007 of this issue, Hazen et al. expand on their seminal 2008 paper on the evolution of minerals, in this case by examining the evolution of the quantity and diversity of clay minerals through time. The timescale of clay genesis spans the age of the solar system, ranging from clay minerals formed by aqueous alteration and shock processes in chondrites, to biogenic clay growth on Earth during the Phanerozoic. Like no other mineral group, clays reflect the interconnection between geosphere, hydrosphere, and biosphere, which, as noted by Hazen, is likely to be vastly better understood, both on Earth and on other planets that have had a hydrosphere (Mars) when geochemists begin to analyze these minerals in earnest.

Mineralogy Trumps Biology

On page 2037 of this issue, Rollin-Martinet et al. show and quantify on an experimental basis that, beyond biologically mediated processes, the existence of a thermodynamic driving force dictates the inexorable evolution of apatite biominerals as observed during bone and enamel maturation. These results probably explain, on a physical basis, the need for bone remodeling in vertebrates for retaining bone mineral role in homeostasis. Biological processes initially precipitate metastable but highly bioactive nonstoichiometric apatites; these crystals are then inexorably driven towards a more stable state (lower Gibbs free energy, largely through decreases in enthalpy) closer to stoichiometry but less bioactive. This work leads to avenues of further research on nanocrystalline apatites in view of more effective bone replacement, and even has implications regarding the origin of life on Earth (as life evolves from carbonate-based shells to more versatile phosphate-based skeletons).

A Post-Seismic Post-Stishovite Phase Transition?

On page 2053 of this issue Asahara et al. examine Brillouin scattering measurements for SiO2 at room temperature, but at high pressure and high differential stress. They find that the stishovite/post-stishovite phase transition occurs at lower pressures at high differential stress, and that the post-stishovite phase is accompanied by a smaller than expected discontinuity in seismic wave speeds. Their results may explain a lack of seismic features in subducted slabs at depths of >1500 km, and hint at the possibility that the stishovite/post-stishovite phase transition may occur in subducted slabs at depths shallower than would be predicted under hydrostatic conditions.

Martian Bacteria?

On page 2105 of this issue Jandacka et al. examine the size distributions of biogenic and inorganic magnetite. Laboratory-cultured bacterial magnetite crystals are observed to evolve to an extreme valued distribution, while uncultured natural samples exhibit a mixture of extreme value and log-normal distributions. Analysis of several populations of magnetite nanoparticles from martian meteorite ALH 84001 show that half of these follow an extreme value distribution, providing very tentative support for a biogenic origin for magnetite growth for such samples.

A new classification of K-feldspars

On page 2115 of this issue Snchez-Muoz et al. for K-feldspars, which improve our understanding of order/disorder relationships among K-feldspars. This study shows that there exists medium-range Si-Al-K or molecular-like order (MRO), involving not only the tetrahedral sites but also the alkali cation position, which cannot be captured by standard diffraction methods. This MRO allows K-feldspars to be divided into two groups, based on number and location of Al atoms in the four-membered rings of tetrahedral sites: microcline and orthoclase, on the one hand, and valencianite and sanidine on the other. This differentiation reveals important structural contrasts that are otherwise hidden from average structure solutions, and suggest to consider "valencianite" as an additional mineral species in the K-feldspars group.

Oxygen Fugacities of the Martian Mantle

On page 2193 of this issue, Papike et al. develop oxybarometers based on vanadium partitioning between four different pairs of phases: V spinel/melt, V/(Cr+Al) spinel/melt, olivine/melt, and spinel/olivine. Except for the spinel/olivine oxybarometer, the models apply over a wide range of fO2 conditions (IW 1 to QFM). And while the spinel/olivine oxybarometer may be restricted in fugacity range (IW 1 to IW + 1), it may still be useful for the characterization of some martian, lunar, and 4 Vesta basalts. For martian meteorite Y98, the oxybarometers indicate a quite reduced fO2 for the martian mantle, at least compared to terrestrial basalts, with a window of IW + 0.8 to IW + 1.6.

The Carbonated Mantle Solidus

On page 2172 of this issue, Shatskiy et al. examine phase relations in the system Na2CO3-MgCO3 at 6 GPa and 900-1400 degrees C, and under dry and hydrous conditions. Their results show that eitolite (Na2Mg(CO3)2), while stable at high pressures, would likely melt for reasonable P-T ascent paths in kimberlites, where eitolite occurs as inclusions in spinel or olivine. But under dry conditions, eitolite partial melting in the presence of magnesite may control the solidus of carbonated peridotite in a subducting slab.

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Notable Papers for October 2013

Review of Alunite and Jarosite Crystal Structures

On page 1633 of this issue, Spratt et al. review the alunite supergroup of minerals (which includes jarosite) that are of current interest due to their discovery on the surface of Mars. Their study carries implications for the presence of acidic, sulfur-rich water on the Martian surface. This review focuses on the structure and vibrational spectroscopic properties of alunite and jarosite, and may allow more rapid and certain identification of such minerals by vibrational spectroscopic methods.

Measuring Water and CO2 in Natural Silicate Glasses

On page 1660 of this issue Lowenstern and Pitcher calibrate a new method for the determination of water contents for natural and experimental silicate liquids (glasses). The authors use an alternative FTIR-based technique called Attenuated Total Reflectance (ATR). This method provides accuracy that is comparable to transmission FTIR, and can be used to analyze spot sizes as small as 5 micrometers. Then, on page 1788 of this issue, Morizet et al. provide a new model for estimating CO2 contents in glasses by a micro-Raman approach. The ATR model of Lowenstern and Pitcher can be applied to glasses containing from 0 to 6 wt% H2O, while the Morizet model can be used for silicate glasses containing up to 16% CO2.

The Foundations of Rome

On page 1669 of this issue, Jackson et al. unlock the secrets of Roman harbor concretes, in their analysis of Al-tobermorite from a 2000-year-old concrete breakwater. The ancient Roman concrete was formulated from a mixture of lime, pumiceous volcanic ash and zeolite-bearing tuff (the volcanic materials are from Flegrean Fields). These components together reacted with seawater to form a binder that consists of poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H). In contrast to modern Portland cement, which produces calcium-silicate-hydrate (C-S-H) binder, the Al-rich C-A-S-H binder is more chemically stable. Jackson et al. examine the chemical and temperature conditions under which the very stable Al-tobermorite crystals formed, and the kinds of starting materials required for its use in modern concrete synthesis.

Where is Water Trapped?

On page 1688 of this issue Trots et al. present a most convincing data set based on a neutron powder diffraction experiment to pinpoint the controversial hydrogen/deuterium positions in the structure of superhydrous phase B. Superhydrous phase B is one of the many nominally hydrous dense magnesium silicates that may act as carriers of H from the surface into the deep Earth. Their work shows that hydrogen is stored in channels formed by edge-sharing octahedra, rather than along the edges of MgO6 octahedra. The exact structure including precise hydrogen positions of hydrous phases is important in order to properly assess the stability and physical properties of hydrogen carrying phases stable in the deep Earth.

Making Granite on the Moon

On page 1697 of this issue Seddio et al. examine a granitic rock fragment from a lunar soil sample recovered from the Apollo 12 mission. Their work shows that the granitic composition is not easily obtained by fractionation of any known lunar basaltic composition, but that a Na-rich KREEP basalt (with 1.2 to 1.4 wt% Na2O in the bulk liquid) could work as a parent magma, or as a protolith of partial melting.

Dissolving the mantle as a solution for surface dwellers' energy and climate problems?

On page 1738 of this issue, Andreani et al. show that the presence of aluminum greatly enhances the rates of dissolution of olivine and concomitant precipitation of serpentine. This is not only of great geological interest since it affects the dynamics of plate tectonics but could potentially have social implications since the serpentinization reaction is a low-energy and carbon-free way of producing H2 and binding CO2.

Mineralogical Control of the Martian Water Cycle

On page 1772 of this issue Leftwich, Bish and Chen examine the reversible hydration-dehydration behavior of Na2Mg(SO4)2nH2O at Mars-relevant conditions. Their work shows that at equatorial latitudes, evaporates of this type may have the ability to absorb H2O from the Martian atmosphere at night (n = 16), and then dehydrate (n = 4) H2O back to the Martian atmosphere during the Martian day, provided that daytime surface temperatures exceed 10 C. At polar latitudes the Na2Mg(SO4)2nH2O phase would remain in the more hydrated (n = 16) state. Their work implies that at equatorial latitudes, such hydration/dehydration reactions could greatly impact the Martian water cycle.

Plagioclase as a Record of Syn-eruptive Water Contents

On page 1779 of this issue Johnson and Rossman examine diffusion rates of H through plagioclase. They find that H diffusion rates are sufficiently fast so as to record only the last few hours of plagioclase growth. This means that deep-seated H contents of a host magma can only be recorded by plagioclase crystals if such crystals are transported at very rapid ascent rates, exceeding 66m/hr. Slower magma ascent rates allow for re-equilibration of plagioclase H contents with ambient magmatic H contents during the last few hours of sub-surface residence, prior to quenching at the surface.

A different spin on the Earth's mantle

On page 1803 of this issue Lyubutin et al. report on a high-pressure Mossbauer study of the (Fe,Mg)2SiO4 polymorph Ringwoodite, believed to be a major constituent in the Earth's mantle between 520 and 670 km. They report the first experimental evidence of an electronic high-spin to low-spin transition for the Fe2+ contained in ringwoodites crystal structure. This has important implications on the physical properties of the Earth's mantle where ringwoodite is stable. Since the high-spin to low-spin-transition affects the volume and thus sound velocities of ringwoodite, this finding has important consequences on the seismic properties of this important mantle mineral.

Monitoring Olivine Growth Rates

On page 1860 of this issue, Grant and Kohn examine the partitioning behavior of P between olivine and silicate liquids (DP). They find that DP is much higher in highly polymerized melts (with low ratios of non-bridging oxygens relative to tetrahedral cations, or NBO/T). Thus, P contents in olivine may change even if P contents in a liquid are constant, as DP responds to changing NBO/T in an evolving melt. Importantly, they also find that olivine P and Al contents are uncorrelated. This finding points the way towards distinguishing rapidly grown crystals (with high P and high Al) from crystals (with high P and low Al) that are grown at near-equilibrium conditions, but from highly polymerized melts.

Notable Articles for August-September 2013

Editorial: Why You Should Publish Your Best Papers in Am Min

On the Size of the Chondrite Parent Body

On page 1379 of this issue, Kevin Righter offers a perspective in a Highlights and Breakthroughs article regarding the recent work by Kimura et al. (2013) on the discovery of an eclogite mineralogy within an unaltered CR Chondrite meteorite. Righter notes that this is at least the second description of an eclogite mineralogy in a chondrite meteorite. Such mineralogies require a parent body radius of at least 1500 km, and as Righter notes, such discoveries may lead to new insights regarding the roles played not just of P and T, but also bulk composition for eclogite formation.

Magmatic Gold

On page 1459 of this issue, Botcharnikov et al. re-examine the effects of temperature, oxygen fugacity, and the identity of sulfide phases on the partition coefficients of Au (DAu) between basalt melts and sulfide phases. New data are presented that indicate an order-of-magnitude difference between the DAu values for monosulfide solid solution/silicate-liquid and sulfide-liquid/silicate-liquid emphasizing the importance of the nature of the sulfide phase present. Temperature, P and fO2 have strong indirect effects on Au partitioning by controlling the identity of the condensed sulfide phase. These authors show that the presence of a monosulfide solid solution (mss) or oxidizing conditions (fO2 close to the sulfide-sulfate transition where sulfur mobility is much higher) favor elevated Au concentrations in magmas.

Dating Planetary Bodies

On page 1477 of this issue, Tartese et al. use NanoSIMS to obtain highly precise Pb/Pb dates on ca. 3.7-3.8 Ga high Ti lunar basalts from the Apollo 11 and Apollo 17 missions. Their work shows that the mineral tranquillityite provides an ideal target for highly precise Pb/Pb dating by ion microprobe. Precision on age dates compared to Rb-Sr and Sm-Nd methods is lower by a factor of 3 to 5, and so provide a more precise timescale for the eruption of high Ti basalts. As these basalts have been used to calibrate chronometers for planet surface ages based on crater density, this work has the potential to add precision to the understanding of surface ages for planetary bodies beyond the Moon.

Making Graphite and Diamonds

On page 1565 of this issue, Tao et al. examine the decomposition of siderite at upper mantle pressures. Magnesite-siderite solid solutions are expected to be the stable form of carbonate as C is subducted into the mantle, but the stability of the Fe end-member of the series had yet to be studied at mantle pressures. Tao et al. find that the decomposition of siderite into magnetite and either graphite or diamond occurs at temperatures that are close to the average mantle geotherm; the cooler subduction geotherms should still retain C in the form of a magnesite-siderite solid solution, but as these carbonates are cycled back into the mantle, siderite decomposition may allow the release of C to form graphite or diamond.

More on Mantle Carbonates

On page 1593 et al. Shatskiy et al. examine melting relationships in the K2CO3-MgCO3H2O system. Their results indicate that if K and C are both held in the form of magnesite and K2Mg(CO3)2 in the upper mantle, then this phase assemblage could control the solidus of carbonated peridotite in the upper mantle. Near-solidus melting of such phase assemblages could explain the origin of ultrapotassic carbonatitic melts that are found as microinclusions in fibrous diamonds from kimberlites worldwide.

Notable Articles for July 2013

A major advance of the bond valence model

On page 1093 of this issue, Brown provides a perspective on a new analysis of bond valence by Bickmore et al. (2013; American Mineralogist, v. 98, p. 340349). This new work provides a means by which bond angles can be at least qualitatively predicted from bond-valence vectors. They key observation is that bond-valence vectors do not always sum to zero, and these sums can be used to infer bond angles in cases where the bonding is asymmetric. This work marks a major advance of the bond valence bond model, and introduces the prospect of our being able to predict bond geometries for even highly complex mineral structures.

Age dating sediments

On page 1106 of this issue Allaz et al. obtain in situ age dates and trace element concentrations of detrital monazite grains. They are able to document dissolution and re-growth of monazite rims during diagenesis, which implies that monazite rim compositions can be used for age dating sedimentary processes, and as might be expected, that whole-grain monazite analyses might not be useful for differentiating sedimentary source areas. Their results also raise cautionary notes about monazite as a storehouse for radioactive waste, given the potential for monazites to dissolve and re-precipitate.

A new method for measuring volatile contents in hard-to-prepare samples

On page 1162 of this issue, King and Larson present a new calibration for micro-reflectance IR spectra for the estimation of water and CO2 contents in volcanic glass samples. The new method allows water contents to be determined for a wide range of glass compositions and volatile contents for samples that are difficult to prepare, either because of fragility, high water contents, or samples that are too small to be doubly polished for transmitted IR measurements.

Magnesite as a host of C in the lower mantle

On page 1211 of this issue diamond anvil experiments conducted by Scott et al. show that magnesite readily forms from MgO and CO2 at 540 GPa and 14001800 K. A key result is that magnesite was able to form even at the highest pressures, whereas CO2 was in a very low partial molar volume form (CO2-V). This work confirms earlier studies that indicate that the volume relationships may allow magnesite to act as the key storehouse of oxidized C in the lower mantle.

High temperatures in the solar nebula

On page 1368 of this issue Ma et al. identify dmisteinbergite, CaAl2Si2O8 from a refractory inclusion recently discovered within the Allende meteorite. This is the first description of this mineral from a meteorite refractory inclusion and, like other phases so derived, provides key evidence about the conditions of the early solar nebula. The authors hypothesize that dmisteinbergite formed during quenching of a silicate melt from temperatures of 12001400 C, probably close to the proto-Sun and later transported outward by and a mechanism that is as yet unclear. Dmisteinbergite also joins nine other refractory silicate minerals, being among the earliest formed mineralogical substances in the solar system.

Notable Articles for May-June 2013

All about monazite

Starting on page 817 of this issue, Hetherington and Dumond introduce a new Special Collection in the journal, titled Versatile monazite: A mineral for resolving complex geological records and challenges in materials science. This new section will feature applications of monazite to geochronology, thermochronology, deformation studies, and the storage of radioactive waste. The section begins with two new papers. First, on page 819, Catlos reviews the use of monazite in geochronology, with a focus on the pitfalls relative to assumptions about the meanings of monazite compositional variations. Then on page 833, Dacheux et al. review monazite as a radwaste matrix generally, with a focus on Pu sequestration and evidence regarding monazites resistance to chemical degradation. One of the long-standing puzzles in nuclear power is how to immobilize the long-lived radioactive isotopes created. Dacheux et al. illustrate the incredible ability of monazite to substitute trivalent and tetravalent actinides, including the transuranic minor actinides. The resistance to metamictization, ease of sintering, and the low leachability of this structure are shown to make it a prime candidate for locking up actinide waste. Look to upcoming issues for additional studies of monazite. This Special Collection is currently open to submissions.

Conditions in the early Solar System

On page 870, Ma et al. describe a new mineral, kangite, found in the Allende meteorite. Kangite is proposed to have formed during an oxidation event from a reduced precursor Sc-Ti oxide, which if discovered, would also constitute a new mineral. Kangite has a very high ratio of heavy to light rare earth elements, providing evidence that the precursor material was ultrarefractory, and so formed from as an early condensate at very high temperatures. The existence of such high temperature condensates has been predicted on thermodynamic grounds (Boynton 1975) and their formation as condensation (rather than evaporation) products is supported by isotopic evidence (Smith et al. 1996).

Mineralogical records of magmatic porosity

On page 888 of this issue, Morse shows that the lever rule can be used to define the solidification history of trapped liquid in an igneous cumulate, or in a melt inclusion inside a crystal. In his model, the two processes differ because the cumulate system closes by failure of communication with the main parent magma, whereas the melt inclusion originates in an evolved sheath surrounding the crystal. For this reason, Morse argues that the melt inclusion cannot represent the parent magma and is destined to evolve further during crystal growth. In either case, the range of the anorthite fraction in plagioclase serves to define the amount of liquid trapped.

The search for habitable environments on Mars

On page 897 of this issue, Hausrath and Olsen use reactive transport modeling to examine aqueous alteration of carbonate (magnesite + siderite) protoliths found on Mars. This work illustrates scenarios of chemical weathering and enrichment in various elements, and shows that it should be possible to detect evidence for near-surface, water-mediated alteration of martian carbonates, since the dissolution-precipitation reactions will leave telltale mineralogical signatures. Moreover, those signatures may also indicate pH or oxidation levels in the reacting fluids. For example, under acidic, low-oxidation state conditions, carbonate protoliths should yield siderite-bearing crusts, while under oxidizing conditions, the surface is preferentially enriched in magnesite and ferric oxide. Acknowledging the complexity of the problem, the authors suggest that future profiles through martian carbonates may be used to test for evidence of aqueous alteration.

Clay minerals as thermometers of diagenesisOn page 914 of this issue, Bourdelle et al. propose a means to improve temperature estimates of clay (illite + chlorite) formation during diagenesis. Their approach requires highly precise nanometer-scale measurements of clay rim and core compositions, which can be used in conjunction with thermodynamic models to yield precise temperature estimates. The nanometer-scale analyses are required as this is the scale at which overgrowths are formed at low temperatures, the chemical equilibrium being approached only at a very fine scale near mineral interfaces.

Cobalt may be more siderophile than Ni

On page 993 of this issue Armentrout et al. use high-pressure experiments to establish an equation of state applicable to the lower mantle. As the authors note, it is not easy to establish metal/silicate partition coefficients at the conditions of the lower mantle due to various experimental challenges. These authors instead explore partitioning by calculating Gibbs free energy contrasts for the equilibrium NiO + Co = Ni + CoO by conducting high-pressure measurements and an equation of state derived from such. Their results show that at pressures <58 GPa CoO + Ni is energetically favorable compared to Co + NiO, but that at >58 GPa, Co + NiO is more stable; this implies that at the pressure of the core mantle boundary (140 GPa), Co may be more siderophile than Ni.

Trading carbonate for phosphate

On page 1066 of this issue, Yi et al. examine carbonate-phosphate substitutions in flourapatite. Much of the world's phosphorus resources are in the form of francolite, a carbonate-rich apatite; yet, how these carbonate groups are incorporated has been a long-standing puzzle. Yi et al. examine the problem using state-of-the-art NMR, FTIR, and first-principles calculations. They show that while a small fraction of carbonate groups do substitute in the hexagonal channels (A-type carbonate), a charge-compensating composite tetrahedron, composed of a planar carbonate group lying on one sloping face and a F ion at the remaining vertex, can replace the PO4 unit (B-type carbonate). They suggest that a particular IR Raman band may prove useful in determining the extent of recrystallization of apatite from bone and teeth.

A new magmatic signal from subducted slabs

On page 1074 of this issue, Hurai et al describe primary magmatic aragonite. It occurs in a carbonatite and carbonated syenite near the HungarySlovakia border. The host rocks contain calcite that is interpreted to represent quenched carbonatite liquid; isotopic ratios indicate that the aragonite formed prior to carbonate-liquid degassing. Aragonite is expected to be stable at pressures >1.22 GPa, i.e., below the Moho in this region. But Mg contents of the carbonate phases are too low to allow equilibration with mantle peridotite. The authors hypothesize that aragonite was stabilized by a transient pressure within the crust (equal to about 186% of lithostatic pressure), generated by degassing. Regardless of the crystallization conditions, isotope ratios of the carbonate phases are consistent with its derivation by metasomatism of mantle wedge materials, due to subducted fluid inputs.

Notable Articles for April 2013

The Bishop Tuff Finally solved?

On page 529 of this issue, Michel Pichavant presents our inaugural Highlights and Breakthroughs article. Pichavant provides context for the work of Evans and Bachmann (2013; Am Min v. 98, p. 271), who present new evidence for strong thermal zonation in the pre-eruptive magma chamber that gave rise to the Bishop Tuff.

The transition from submarine to subaerial volcanism?

On page 582 of this issue, Li et al. report the occurrence of quartz nano-crystals that occur in amorphous chert within a 2.48 Ga banded iron formation (BIF) in western Australia (Dales Gorge). The quartz nanocrystals are interpreted to have formed as pyroclasts and deposited as dust within the evolving BIF. This work shows that subaerial volcanism occurred at least as early as 2.48 Ga the earliest known occurrence of such activity.

Can we hear the disaster coming?

On page 609 of this issue, Salje et al. investigate the collapse of porous mineral aggregates upon uniaxial compression. Their experiments reveal that the sample collapse is following self-organized avalanche criticality where the probability of an event of a given energy follows a power law. A high correlation between the occurrence of large avalanches and fore-shocks in porous samples opens the possibility to predict in principle a major collapse through acoustic detection of crackling noise.

Oxidation states in martian basaltic liquids

On page 616 if this issue, Righter et al. show that Fe3+/Fe2+ ratios in high Fe martian basaltic rocks are much lower than the Fe3+/Fe2+ ratios in terrestrial samples that equilibrate at comparable fO2 conditions. This finding not only has implications for mineral stability, but impact, for example, mantle potential temperatures based on Fe-Mg partitioning in mantle derived liquids.

Can we extract quantitative information from unpolarized IR spectra?

On page 689 of this issue, Tony Withers presents a rigorous analysis of the use of unpolarized infrared (IR) spectra for the quantitative analysis of absorbing species. The Beer-Lambert law, which links IR absorbance to concentration of an absorbing species, cannot strictly be applied to unpolarized IR radiation in birefringent media. Polarized IR spectroscopy is often not possible, however, and using unpolarized IR in general introduces significant errors for strongly absorbing samples. Withers deduces a theory through which those errors can be avoided.

How to name your garnet

On page 785 of this issue, Grew et al. review the nomenclature of the garnet supergroup (which includes all minerals isostructural with garnet). The new nomenclature identifies 21 compositional end-members. Most of these are trace constituents, with only six comprising >20 mol% of any known mineral. Fans of hibschite, however, will be highly disappointed.

Notable Articles for February-March 2013

Spherulites are interesting -- Who knew?

On page 304 of this issue, Seaman examines water contents of spherulites from three different rhyolite flows from southern Arizona. Water contents generally increase outward along the length of spherulite crystals. Water is not partitioned into the growing crystal structures, but is captured as fluid inclusions. Spherulites exhibit either blade-like or needle shapes, which vary as a function of cooling rate. Water contents also vary discontinuously along spherulite grains, and so likely monitor the build-up and occasional degassing of ambient melt.

How glass turns to clay

On page 319 of this issue, Cuadros et al. conduct experiments on volcanic glass to compare how clays are affected by fluid compositions and biogenic processes. Fluids as different as natural spring and hypersaline waters are reacted with basaltic glass. For the most part, clay compositions are controlled by glass compositions, not water chemistry. Geochemical relationships also show that clays nucleate within the glass, absorbing Al, Mg, and Fe form adjacent regions of glass, rather than being nucleated within neighboring fluids. Microbes, however, can affect clay compositions by trapping precursor glass in biofilms whereas more extreme fluid compositions may be trapped, and so affect clay composition.

Rising through the mantle

On page 335 of this issue, Kato et al. investigate phase transitions of Al2O3 at high pressures. Their work shows that the transition from a Rh2O3(II)-type to a CaIrO3-type structure occurs at ~170 GPa, i.e., pressures greater than the ~136 GPa that occur at the base of Earths mantle. Kato et al. thus reject the idea that subducted high-Al, anorthositelike compositions might be negatively buoyant in the lower mantle and so contribute mass to the D" layer. The failure to reach the Rh2O3(II)-CaIrO3 phase transition instead means that Al2O3-phase-bearing materials will have a bulk density similar to ambient pyrolite in the lower mantle, and so would be advected upwards, rather than being stored at the core-mantle boundary.

Crystal stability and bond valence models

On page 340 of this issue, Bickmore et al. introduce an expansion of the vectorial bond-valence model (VBVM) and apply it to crystals of simple oxides. A key aspect of the VBVM is that the vector sum of bond valences incident to an ion should match some ideal value, similar to the way in which sums of electrostatic valence bond strengths are applied in Pauling's rules to test for stability. Bickmore et al. show that ideal vectorial valence sums can be predicted, even in cases where electronic structure effects such as lone pairs distort the coordination sphere, based on a quantity they call the minimum coordination number, which is the the absolute value of atomic valence divided by the valence of the strongest bond that reaches an ion. Crystals in which vectorial valence sums deviate strongly from ideal values tend to be unstable or metastable, so this model can be used to rationalize and predict phase stability.

Radiation damage in zircons

On page 350 of this issue, Ketcham et al. report on a detailed study of alpha recoil tracks and fission tracks in zircon. Using the mathematics of percolation, they show that alpha recoil tracks percolate at doses two orders of magnitude lower than previously thought, i.e., at doses too small to be detected by swelling or metamictization, whereas fission tracks percolate near the dose levels required to see such effects. Perhaps counterintuitively, the percolation of alpha recoil tracks appears to minimize He diffusion, whereas the percolation of fission tracks is linked with poor He retention.

Eclogites in a chondrite meteorite -- Implications for a large parent body

On page 387 of this issue, Kimura et al. report the first finding of a high-pressure mineral assemblage in clasts within a chondrite meteorite. Olivine is the dominant phase, but unlike other meteorites, it is accompanied by Na-Al-rich pyroxene (omphacite) and garnet, which also coexist with orthopyroxene and often phlogopite, a mineralogy that is akin, although not identical, to terrestrial eclogites. Subsolidus thermobarometers yield equilibration temperatures of 940-1080 C, and pressures of 2.8 to 4.2 GPa. These pressures are too low to represent shock conditions recorded in shock veins, and the pyrope-rich garnet and Na-Al-rich pyroxenes are hardly found in shocked meteorites in any case. A possible explanation is that these eclogite clasts formed in a planetary interior, although impact origin is not totally rejected. One possibility for the origin of these clasts is thus a parent body with a chondrite composition, and a minimum radius of 1500 km, larger than any existing asteroid. If this is the case, then the range of pressure for asteroid formation is considerably greater than previously considered.

A record-high volcanic fO2?

On page 417 of this issue, Mullen and McCallum show evidence for very high oxygen fugacities for the Coleman Pinnacle andesite flow at Mt. Baker, Washington. The andesite contains an unusual assemblage of Fe-Ti-Mg oxides that includes pseudobrookite, a relatively rare mineral oxide, more commonly occurring in alkalic volcanic rocks. Their textural and mass balance considerations, and the high Mg no. of coexisting ilmenite-titanomagnetite pairs, suggest that pseudobrookite at Mt. Baker is not a low-temperature decomposition product, but rather the result of precipitation from a magma at very high fO2. Oxygen fugacities for the pseudobrookite-bearing samples are in the range of NNO+1.5 to NNO+1.75, whereas psuedobrookite-absent samples yield fugacities of NNO+0.75 to NNO+1.0. The pseudobrookite-bearing andesite has the highest abundance of fluid-mobile elements in Mt. Baker lavas and minimal crustal input. These features and the accompanying high water contents indicate that the high fO2 conditions are inherited from a hydrated mantle source.

Honors for Charlie Prewitt and Luca Bindi

On page 463 of this issue Shuvalov et al. describe a new mineral named in honor of Charles Prewitt, while on page 470 Garavelli et al. describe yet another new mineral, named for Luca Bindi. The former is green and tabular, and the latter is hexangular and platy, but we have it on high authority that neither mineral carries a resemblance to either honoree (as is so often said: debonair is the scientist who studies minerals). Type localities are appropriately fumarolic. Congratulations, Charlie and Luca.

Notable Articles for January 2013

Conditions of shock metamorphism at Vredefort Dome, South Africa

On page 53 of this issue, Erickson et al. (2013) use microstructures in shocked detrital zircon grains to gauge the pressures of impact metamorphism and deformation history at the Vredefort Dome in South Africa, which at 2 Ga is the oldest impact site yet identified on Earth. Zircon samples are an especially useful target because they are able to survive impact, as well as later uplift, erosion, and sedimentary transport, allowing long-lasting preservation of impact effects. This work shows that impact pressures at Vredefort Dome are between 20 and 40 GPa, as planar fractures develop only above the lower of these pressures, while zircon converts to reidite (not observed in their shocked crystals) at the upper pressure limit.

Origin of Banded Iron Formations

On page 85 of this issue, Sun et al. (2013) examine inter-layered growths of Fe-Si oxides from hydrothermal vents within the Lau Basin. Their work shows that biogenic Fe hydroxides can easily bind to dissolve silica, leading to an intimate mixture of Fe-Si oxides. These Fe-Si oxides grow in two stages. The first involves the formation of minerals such as ferrihydrite, as bacteria oxidize Fe2+ to Fe3+ in the process of fixing dissolved CO2 from hydrothermal fluids. This initially loose network of ferrihydrite crystals becomes increasingly dense, to the point where hydrothermal fluids no longer freely circulate, which allows Si-supersaturated solutions within the structure to precipitate opal. The result is not a banded structure, but instead an intimate mixture of Fe-Si oxides, which implies that the separation of Fe- and Si-rich layers in banded iron formations occur by later, digenetic processes, rather than by direct biogenic-mediated precipitation of alternating Fe- and Si-rich layers.

Limits to Ti-in-Quartz Thermometry From Observations of Dis-equilibrium

On page 98 of this issue, Vasyukova et al. (2013) investigate trace element zoning by cathodoluminescnece in quartz crystals from porphyritic, mineralized granitic rocks. Quartz crystals yield oscillatory zoning patterns for Ti, with sharp compositional boundaries relative to Ti, but gradational boundaries relative to Al contents. They also examined clusters of quartz crystals, where individual grains exhibit very different Ti contents. Several of their observations suggest that Ti variations are unlikely controlled by temperature (T): (1) calculated T estimates from Wark and Watson (2006) range from 791 to 889 C, which are considered too high for late stage porphyritic intrusions; (2) unreasonably high T variations -- as great as 140 C -- are required to explain oscillations in Ti; and (3) sharp Ti oscillation boundaries imply seemingly unlikely sharp variations in T, especially in a late stage, non-convecting magma. The authors conclude that fluid loss increases Ti activity in the residual melt, and that oscillations in Ti concentrations reflect fluid loss events, rather than a record of thermal history.
Wark, D.A., and Watson, E.B. (2006) TitaniQ: a titanium-in-quartz geothermometer. Contributions to Mineralogy and Petrology, 152, 743-754.

Deep Subducted Crust and a Repository for Large Ion Lithophile Elements in the Lower Mantle

On page 207 of this issue, Kawai and Tsuchiya (2013) show from first-principles calculations that K-hollandite II (KAlSi3O8) replaces K-hollandite I at high pressures, as the stable K aluminosilicate. At mantle temperatures, K-hollandite II would be stable from near the top of the lower mantle (at 700 to 770 km) down to the core mantle boundary. In the event that continental materials (alkali feldspars), perhaps in the form of subducted sediments, can survive to be subducted to the lower mantle, the K-hollandite II phase could then provide a significant repository for K and other large ion lithophile elements. This work further shows that solid solution effects of K-hollandite II with Na-hollandite II have only a negligible effect on K-hollandite II stability and that Na-hollandite is a metastable phase at lower mantle pressures.

The Effect of Metamorphism on Zircon Age Dates

On page 219 of this issue, Vorhies et al. (2013) conduct SIMS U-Pb depth profiles of zircons in metamorphic rocks from the type section of Barrovian metamorphism in Scotland. Metamorphic grade for their samples ranges from the chlorite to sillimanite + K-feldspar. They find that even at the highest grades of metamorphism, only the very outermost margins of zircon rims (<1 micrometer depth) have their ages reset by metamorphism. Zircon interiors thus mostly record pre-metamorphic histories (pre-470 Ma), where in this case zircon interiors yield age dates of mostly 600 to 2000 Ma. In the outermost (<1 micrometer) zones, a range of age dates are recovered that reveal a complex history of decompression melting, later intrusion of granitic rocks, volcanism, and mineralization, ranging from 470 to 250 Ma. Only those zircon rims formed in the sillimanite zone contained rims that would be wide enough (>20 micrometers) to yield peak metamorphism age dates by spot analyses.

Thermal History of the Bishop Tuff, From Mineral-Mineral Equilibria, Through a Metamorphic Lens

On page 271 of this issue, Evans and Bachmann (2013) re-evaluate published analyses of mineral compositions from the Bishop Tuff, but in contrast to prior studies, they apply a strikingly simple yet important tool for the assessment of equilibrium from metamorphic petrology: Roozeboom diagrams. These authors compare XMg (mole fraction of Mg in phase (small greek phi, your browser may vary)) for ilmenite, biotite, orthopyroxene, magnetite, and clinopyroxene. Invariant and wide-ranging XMg in the pyroxenes disallows their equilibrium with other crystalline phases, but monotonic trends in plots of XMgilm vs. XMg, with plausible (0, 0) and (1, 1) intercepts allow equilibrium between all other crystalline phases. Evans and Bachmann conclude that ilmenite-magnetite pairs faithfully record magma temperatures of a water saturated high SiO2 rhyolite magma, at ~700C, which later mixed with a hotter (~800C) recharge magma. This recharge event donated to the resident high SiO2 magma the (now disequilibrium) pyroxenes, and compatible elements such as Mg and Sr, and caused melting of resident sanidine, thereby contributing Ba and K to ambient (and mixed) high SiO2 melt. This work thus supports the recharge-partial melting model of Hildreth and Wilson (2007) and Wark et al. (2007), which explains some otherwise troublesome trace element systematics.
Hildreth, W.S. and Wilson, C.J.N. (2007) Compositional Zoning in the Bishop Tuff. Journal of

Petrology, 48, 951-999. Wark, D.A., Hildreth, W., Spear, F.S., Cherniak, D.J., and Watson, E.B. (2007) Pre-eruption recharge of the Bishop magma system. Geology, 35, 235-238.

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