The redox state controls many important chemical reactions in natural systems, including the reactions in the deep interior of the Earth. The oxidation state of iron has been extensively used to infer the redox state. It has been well understood that the upper mantle has reducing redox conditions and thermodynamic studies have predicted even more reducing conditions in the lower mantle. However, recent studies at pressure-temperature conditions related to the topmost lower mantle have shown that about 60% of iron in the dominant lower-mantle mineral, magnesium silicate perovskite, is ferric. This is very surprising because only 2% of iron is ferric in upper-mantle minerals. If this result is extrapolated to the deep lower mantle, it would imply that the lower mantle is chemically distinct from the upper mantle, which may conflict with recent seismic tomography evidence of mixing between the upper and lower mantle.
Our preliminary work showed that the oxidation state of iron in mantle silicate perovskite is strongly coupled with the electronic configuration of iron at high pressure. As recent studies have shown that the electronic configuration of iron in mantle minerals changes with depth, our preliminary work implies that the oxidation state of iron may change with depth as well. In addition, aluminum in silicate perovskite can influence the oxidation state of iron. In this project, we will combine experiment (Shim) and theory (Morgan) to measure how the oxidation state of iron in lower-mantle silicate perovskite changes with depth and composition. In the experimental effort, we will measure the oxidation state of iron by controlling the redox state of the sample chamber to that of the lower mantle at high pressure-temperature. We use ab-initio based techniques, which solve the fundamental quantum mechanical equations for the material, to predict reaction energies and volumes as a function of composition, pressure, and temperature. This joint effort will provide graduate students and postdocs with new opportunities to combine experimental and theoretical approaches.
