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Application of microprobe-based flank method analysis of Fe3+ in garnet of North Qilian eclogite and its geological
Xiaoli Li1, Shuguang Song1, Lifei Zhang1
1The Key Laboratory of Orogenic Belts and Crustal Evolution MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China.
Abstract:
A newly developed microprobe-based methodology (the Flank Method) for Fe3+/ΣFe quantification has been successfully applied to some natural garnets from the North Qilian eclogites by JEOL JXA-8100 microprobe at Peking University. The results demonstrated an obvious discrepancy in comparison with the outcomes by conventional stoichiometric calculations. This methodology allows to measure the Fe3+/ΣFe ratio and perform elemental analyses simultaneously in the same condition. Accurate in-situ measurement of Fe3+ content in garnet may bring certain impact on the garnet-based P-T estimation. According to the compositional zonation displayed in the studied eclogitic garnets from North Qilian, a prograde metamorphic PT path from 19.5 kbar, 520 °C to 22 kbar, 600 °C was reconstructed. More interestingly, the measured Fe3+/ΣFe ratios in garnets decreasing from core to rim may probably imply that the oxygen fugacity (fO2) declines with the depth of the subduction zone.
