High-Pressure XAFS Measurements of the Coordination Environments of Fe2+
and Fe3+ in Basaltic Glasses
Abstract
We investigated pressure-induced changes in the coordination
environments of Fe2+ and Fe3+ in
basaltic glasses based on the Fe K-edge X-ray absorption fine structure
(XAFS) analyses for both XANES and EXAFS regions. Upon compression from
1 bar to ~15 GPa, the Fe2+-O bond
length remained similar, suggesting that the average coordination number
of Fe2+ increased from ~5 to 6. On the
other hand, the Fe3+-O bond was remarkably elongated,
which indicates that Fe3+changed from 4-fold to 6-fold
coordination. Above 15 GPa, both Fe2+-O and
Fe3+-O bond lengths decreased smoothly, suggesting
minor changes in their coordination numbers. The data also showed that
both Fe2+ and Fe3+ remained in the
high-spin state up to 83 GPa and 60 GPa, respectively, in the basaltic
glasses. These compression behaviors of the Fe2+-O and
Fe3+-O bonds support that Fe2+
disproportionates into Fe3+ and metal Fe in a deep
magma ocean.