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Redox state at ultrahigh-pressure metamorphism: Constraint from the Chinese Continental Scientific Drilling eclogites



In order to estimate the fluid composition, C-species of the CCSD (Chinese Continental Scientific Drilling) eclogites were investigated. Graphite is always recognized with pyrite in the eclogites, suggesting their growth from the H2O-rich (CO2-poor) fluids released during metamorphism. The studied eclogites contain vacancy (CaEs component =Ca0.50.5AlSi2O6) in clinopyroxene. Therefore, Fe3+ content in clinopyroxene cannot be calculated based on their chemical composition determined by an electron probe micro analyzer. We estimated Fe3+Fe based on the Mössbauer analysis and calculated the pressure–temperature (P–T) conditions from the assemblage of phengite-garnet-clinopyroxene-(coesite) as P = 3–4 GPa, and T = 650–780°C. The P–T-values obtained in this study show lower values than those which were previously reported elsewhere for the CCSD eclogites, suggesting that the effect of Fe3+ estimation is significant.