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, 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.
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.
Highlights & Breakthroughs
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 hidden—and may remain so absent detailed and comprehensive field and petrologic studies.
Predicting Mining Accidents
On page 3 of this issue, Ulrich Bismayer provides an overview of a paper that we highlighted from last month’s 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.
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.
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 Earth’s core, there is a recognition that the Moon’s 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 Moon’s formation.
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.
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.
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.
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 element’s 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. Karato’s 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.
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.