Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa

Abstract

Experiments with peridotite minerals in simple (MgO^Al2O3^ SiO2, CaO^MgO^SiO2 and CaO^MgO^Al2O3^SiO2) and natural systems were conducted at 1300^15008Cand 6^10 GPa using a multi-anvil apparatus.The experiments in simple systems demonstrated consistency with previous lower pressure experiments in belt and piston^cylinder set-ups. The analysis of spatial variations in pyroxene compositions within experimental samples was used to demonstrate that pressure and temperature variations within the samples were less than 0 4 GPa and 508C. Olivine capsules were used in natural-system experiments with two mineral mixtures: SC1 (olivineþ high-Al orthopyroxeneþ high-Al clinopyroxeneþ spinel) and J4 (olivineþ low-Al orthopyroxeneþ low-Al clinopyroxene þ garnet). The experiments produced olivineþ orthopyroxeneþ garnet clinopyroxene assemblages, occasionally with magnesite and carbonate-rich melt. Equilibrium compositions were derived by the analysisofgrainrimsandevaluationofmineralzoning.Theywere comparedwith our previous experimentswith the same startingmixtures at 2 8^6 0 GPa and the results from simple systems.The compositions of mineralsfrom experimentswithnaturalmixturesshowsmoothpressure and temperature dependences up to a pressure of 8 GPa.The experiments at 9 and 10 GPa produced andradite-rich garnets and pyroxene compositions deviating from the trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene andCa in clinopyroxene). This discrepancy is attributed to a higher degree of oxidation in the high-pressureexperimentsandanorthopyroxene^high-P clinopyroxene phase transition at 9 GPa. Based on new and previous results in simple and natural systems, a new version of the Al-in-orthopyroxene barometer is presented. The new barometer adequately reproduces experimental pressures up to 8 GPa.