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 1300–1500°C and 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 50°C. Olivine capsules were used in natural-system experiments with two mineral mixtures: SC1 (olivine + high-Al orthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine + low-Al orthopyroxene + low-Al clinopyroxene + garnet). The experiments produced olivine + orthopyroxene + garnet ± clinopyroxene assemblages, occasionally with magnesite and carbonate-rich melt. Equilibrium compositions were derived by the analysis of grain rims and evaluation of mineral zoning. They were compared with our previous experiments with the same starting mixtures at 2·8–6·0 GPa and the results from simple systems. The compositions of minerals from experiments with natural mixtures show smooth pressure 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. higher Al in orthopyroxene and Ca in clinopyroxene). This discrepancy is attributed to a higher degree of oxidation in the high-pressure experiments and an orthopyroxene–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.