George E. Harlow
Department of Earth and
Multianvil experiments have been carried out from 3.0 to 11 GPa and from 1100 to 1500°C on an equimolar mixture of natural diopside (CaMgSi2O6) and F-rich phlogopite (K1.7Na0.24Mg5.6Al2.4Si5.8O20 [F1.7OH2.3]) to examine the behavior of K and F at high P and T in this system. As the first extensive experiments in the KCMASH composition space to include fluorine, the subsolidus crystallization of the perovskite KMgF3 and F-bearing clinohumite and chondrodite at high pressures, up to 10 GPa and 1400ºC, is noteworthy. The compositions of crystalline phases produced in the experiments vary systematically with P and T. In clinopyroxene: Kcpx (KAlSi2O6) content in clinopyroxene increases with increasing pressure at pressures above 5 GPa without a noticeable temperature effect; CaTs (CaAl2SiO6) content is negatively correlated with P above 3 GPa and is positively correlated with T. In amphibole – an M4K-substituted potassic richterite, ideally K(KCa)Mg5Si8O22(OH,F)2: K content is positively correlated with P; Al and F content decrease with P; and F content is positively correlated with T and is lowered by coexisting KMgF3. In garnet: Si and Na content increases upon increasing P; Ca-Mg systematics are complicated by growth zoning of crystals. All of the above trends may merit further calibration in geobarometry above 6 GPa, where published Al-in-Opx and Opx-Cpx barometers fail for these experiments. Although fluorine content in hydrous minerals decreases upon increasing P, fluorine enlarges the high P-T stability field of the hydroxy-minerals, so F content, even at low levels, must be considered in modeling phase stabilities for mantle assemblages. However, the strictly subsolidus paragenesis of humites in these experiments adds to the evidence that mantle humites form via metasomatic rather than magmatic interactions.
Keywords: potassic richterite, potassic clinopyroxene, KMgF3, clinohumite, chondrodite, fluorine, experimental study, high pressure.
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Copyright © 2002 by the Mineralogical Society of America