Fe3+ content of experimental pyroxenes and melt analyzed with in-situ synchrotron Mössbauer spectroscopy

The redox variations during magmatic processes are mainly controlled by the partition coefficients of Fe3+ and Fe2+ between minerals and melts (DFe3+, DFe2+ and DFe3+/DFe2+). However, the mineral/melt DFe3+ and DFe2+ are sparsely reported due to the difficulties in determining Fe3+ content of the minerals in quenched experimental run products. Recent progress in the use of synchrotron radiation sources has developed new frontiers in the study of Fe3+ partitioning between minerals and melts. In this study, we analyzed Fe3+ content of experimental pyroxenes and melt with the in-situ synchrotron Mössbauer spectroscopy using a beam size as small as < 10 μm and determined accurate DFe3+ and DFe2+ for orthopyroxene (opx), clinopyroxene (cpx) and spinel (spl) crystallized at the equilibrium P and T of 1.3 GPa and 1200 °C. The results show that the DFe3+/DFe2+ are < 1 for opx and > 1 for cpx and spl. It is, therefore, expected that during partial melting of mantle spinel peridotite, the consumption of opx will decrease Fe3+/FeT and fO2 of the magma, while the consumption of cpx and spl has the opposite effect. In contrast, the crystallization of opx from basaltic magma tends to increase Fe3+/FeT and fO2 of the magma.