The electronic structure of iron in rhyolitic glass at high pressure

The physical properties of silicate melts within the Earth's mantle affect the chemical and thermal evolution of the Earth's interior. The behavior of iron in mantle melts is poorly understood, but can be experimentally approximated by iron-bearing silicate glasses. Previous studies have conflicting conclusions on whether iron in lower mantle silicate melts goes through a high-spin to low-spin transition [1-3]. Additionally, the average coordination environment of iron in glasses is poorly constrained. XANES experiments on MORB glasses have demonstrated that both four and six-fold coordinated iron may exist in significant amounts regardless of oxidation state [4] while conventional Mossbauer experiments have observed five-fold coordinated Fe2+ with small amounts of four and six-fold coordinated Fe2+[5]. In an attempt to resolve these discrepancies, we have measured the hyperfine parameters of iron-bearing rhyolitic glass up to ~115 GPa in a neon pressure medium using time-resolved synchrotron Mössbauer spectroscopy at the Advanced Photon Source (Argonne National Laboratory, IL). Our spectra are well explained with a three-doublet model: two high-spin Fe2+-like sites with distinct quadrupole splittings and similar isomer shifts and one high-spin Fe3+-like site. Our results indicate that iron experiences changes in coordination with increasing pressure without undergoing a high-spin to low-spin transition. With the assumption that silicate glasses can be used to model structural behavior in silicate melts, our study predicts that iron in chemically-complex silica-rich melts in the lower mantle likely exists in a high-spin state. References: [1] Nomura, R. et al., Nature 473 (2011). [2] Gu, C. et al., Geophys. Res. Lett. 39 (2012). [3] Mao, Z. et al., Am. Mineral. 99 (2014). [4] Wilke, M. et al., Chem. Geology 220 (2005). [5] Cottrell, E. and Kelley, A.K., Earth Planet. Sci. Lett. 305 (2011).