Abstract
The relatively low-variance mineral assemblage of a talc-kyanite eclogite from Dabie Shan enabled application of both conventional geothermobarometers (garnet-clinopyroxene geothermometer and the garnet-omphacite-phengite geobarometer) and a multi-equilibrium method to determine peak P-T conditions (THERMOCALC, “average PT”). The results were highly discrepant: 840 °C / 31.1 kbar vs. 590 °C / 29.8 kbar. Mossbauer spectroscopy showed that Fe3+/Fetotal in omphacite was significantly higher than the value obtained from standard formula recalculation. When the activities were corrected for Fe endmembers, geothermobarometry gave consistent results (606 °C / 31.3 kbar vs. 585 °C / 30.8 kbar). These are close to those obtained earlier by “average PT”, confirming the robustness of the multi-equilibrium approach. The high Fe3+ concentration in omphacite is best explained using a Ca-eskola endmember Ca0.5[]0.5AlSi2O6 and allowing corresponding vacancies in the omphacite structure.
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