Probing the oxidation state of iron in the deep mantle using high P,T Mössbauer spectroscopy

Abstract:

The bulk of the Earth’s interior is not directly accessible, yet redox processes occurring deep within drive many of the events observed at its surface. Laboratory studies of the relevant minerals at the appropriate pressure and temperature conditions have been pivotal in advancing our understanding of the Earth’s interior. Iron plays an important role because it is the only major element with multiple electronic configurations (oxidation and spin state); however most of our knowledge regarding the oxidation state of iron in deep mantle phases is based on measurements of samples quenched from high temperature and sometimes also from high pressure. Mössbauer spectroscopy has been a key player in such studies and results on quenched samples have shown that the oxidation state of iron varies considerably through the mantle, from predominantly ferrous iron in upper mantle and transition zone phases to roughly 50% ferric iron in silicate perovskite (bridgmanite) due to the strong affinity of ferric iron for that phase in the presence of trivalent aluminium. Mössbauer measurements at pressures and temperatures along the geotherm are impractical using conventional radioactive sources due to their low brightness and the extremely limited possibilities for focusing in a laboratory setting. To address these limitations, we have developed an energy domain Synchrotron Mössbauer Source (SMS) on beamline ID18 at the European Synchrotron Radiation Facility that enables rapid collection of high quality energy domain Mössbauer spectra, and coupled with a portable double-sided laser heating system, SMS spectra can be collected on iron-containing mantle phases at pressures and temperatures up to those near the Earth’s core in only a few hours (or less). The presentation will showcase recent results documenting in situ determination of iron oxidation state in lower mantle phases and discuss their implications for redox sensitive processes taking place throughout the Earth’s interior.