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Micro-X-ray absorption near edge structure determination of Fe3+Fe in omphacite inclusion within garnet from Dabie eclogite, East-Central China

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Abstract 

The Fe K-edge micro-X-ray absorption near edge structure (XANES) spectra of omphacite included within garnet from the Dabie eclogites, east-central China, were measured to estimate the oxidation state of iron in omphacite. The micro-XANES analyses of omphacite inclusions within garnet yield higher Fe3+Fe ratios of 0.18 to 0.59 than those of 0.00 to 0.31 calculated by charge balance from electron microprobe analyses. The estimated temperatures of garnet-clinopyroxene pairs from 10 eclogites using Fe3+ and Fe2+ contents calculated by charge balance are 603 to 1078°C except one sample. The temperatures calculated using micro-XANES-measured Fe3+ and Fe2+ contents are 530–918°C except one sample. Along the north–south cross-section in the study area, the estimated temperatures based on the charge balance calculation of Fe3+ tend to be identical or higher than temperatures estimated based on the calculation of Fe3+ using the XANES technique. Seven of the micro-XANES measurements give approximate temperatures from 500°C to 700°C, and three of the measurements give temperatures of more than 800°C. Those three samples contain higher XGrs corresponding to the distribution coefficient of garnet-clinopyroxene pairs. The pressure and temperature results estimated by the garnet-clinopyroxene-phengite geothermobarometers are mostly in subsolidus conditions, suggesting that there is no extensive melting in eclogite. The present study represents a new temperature calibration based on Fe3+ estimates in omphacite inclusion within garnet based on the XANES analysis, which is taken into account.

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