High‐Pressure XAFS Measurements of the Coordination Environments of Fe 2+ and Fe 3+ in Basaltic Glasses

Abstract The X‐ray absorption fine structure (XAFS) spectrum is sensitive to the local structure of a specific element of interest and a useful tool for elucidating changes in the coordination environments and spin states of Fe 2+ and Fe 3+ . However, in situ XAFS study of Fe 2+ and Fe 3+ in a silicate glass has not been conducted under high pressures due to experimental difficulties. Here we investigated pressure‐induced changes in the coordination environments of Fe 2+ and Fe 3+ in basaltic glasses based on the Fe K‐edge XAFS analyses for both X‐ray absorption near edge structure and extended X‐ray absorption fine structure (EXAFS) regions. Upon compression from 1 bar to ∼15 GPa, the average Fe 2+ ‐O bond length determined by the EXAFS analyses remained similar, suggesting that the mean coordination number of Fe 2+ increased from ∼5 to 6, which is also supported by a decrease in the pre‐edge intensity. On the other hand, the Fe 3+ ‐O bond was remarkably elongated, which indicates that Fe 3+ changed from 4‐fold to 6‐fold coordination. Above 15 GPa, the coordination numbers of both Fe 2+ and Fe 3+ further increased, which is inferred from comparisons of the bond lengths with those of reference melt and minerals. In addition, the bond lengths did not show an anomalous reduction, indicating that both Fe 2+ and Fe 3+ remained in the high‐spin state up to 83 and 60 GPa, respectively, in the basaltic glasses. These compression behaviors of the Fe 2+ ‐O and Fe 3+ ‐O bonds support that Fe 2+ disproportionates into Fe 3+ and metal Fe in a deep magma ocean.