Determination of Fe3+ and Fe2+ Partition Coefficients between Pyroxenes and Basaltic Melt with in-Situ Synchrotron Mössbauer Spectroscopy

The oxygen fugacity (fO2) of a mineral−melt system is reflected by Fe3+/FeT ratio (FeT = Fe3+ + Fe2+) of the melt. During magma generation and evolution, the variations in melt Fe3+/FeT is mainly controlled by the partition coefficients of Fe3+ and Fe2+ between silicate minerals and melts (D_Fe³⁺, D_Fe²⁺ and D_Fe³⁺/D_Fe²⁺), although the bulk Fe3+/FeT of the system, the mineral assemblage, and the proportions of mineral(s) to melt also play a role. Therefore, an accurate understanding of mineral/melt D_Fe³⁺ and D_Fe²⁺ is required to quantify Fe3+/FeT and fO2 variations during magmatic processes. However, the mineral/melt D_Fe³⁺ and D_Fe²⁺ are sparsely reported due to the difficulties in determining the 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 experimentally synthesized pyroxenes and melt with the in-situ synchrotron Mössbauer spectroscopy using a beam size < 10 μm and determined accurate D_Fe³⁺ and D_Fe²⁺ for orthopyroxene (opx), clinopyroxene (cpx), and spinel (spl) crystallized at the equilibrium pressure and temperature of 1.3 GPa and 1200 °C. The obtained D_Fe³⁺ values are 0.62 (±0.02) for opx, 0.96 (±0.03) for cpx, and 6.79 (±0.28) for spl. The corresponding D_Fe²⁺ are 1.05 (±0.03) for opx, 0.53 (±0.01) for cpx, and 2.96 (±0.08) for spl. The D_Fe³⁺/D_Fe²⁺ ratios, calculated from paired D_Fe³⁺ andD_Fe²⁺ values, are 0.59 (±0.02) for opx, 1.85 (±0.07) for cpx, and 2.29 (±0.04) for spl. It is, therefore, expected that during partial melting of spinel lherzolite in a closed system, the consumption of opx will decrease Fe3+/FeT and f_O₂ of the magma. In contrast, the consumption of cpx and spl has the opposite effect. A preliminary modeling, based on these partition coefficients, shows that partial melting of a sub-arc mantle with Fe3+/FeT of 3.7% cannot account for the elevated Fe3+/FeT of primitive arc basalts, suggesting that arc basalts inherited the oxidizing signature from intrinsically oxidized mantle sources.