The effect of Fe spin crossovers on its partitioning behavior and oxidation state in a pyrolitic Earth's lower mantle system

Abstract

Geophysical interpretations of the Earth's interior and its dynamics are significantly influenced by phase transitions of the constituting minerals and their chemical compositions. Pressure induced Fe spin crossovers in the main mineral phases of the Earth's lower mantle, Mg–Fe silicate perovskite and ferropericlase, have been suggested to influence Fe partitioning resulting in separate layers with distinct physical properties. However, previous results remain ambiguous regarding the exact effect of Fe spin crossovers and the actual transition pressures. We observe here a continuous decrease of the Fe2+–Mg partition coefficient KD between silicate perovskite and ferropericlase from 25 GPa to 79 GPa in a pyrolitic Earth's lower mantle system. At about 97 GPa the KD significantly increases with an accompanied decrease of the Fe3+/ΣFe ratio in perovskite, which therefore leads to an amplified change in the Fe2+KD. We conclude that the Fe2+ high-spin to low-spin crossover in ferropericlase and the Fe2+ high-spin to intermediate spin crossover in perovskite at mid-lower mantle pressures (30–80 GPa) exert no control on KD, but the Fe2+ intermediate-spin to low-spin crossover in silicate perovskite at about 100 GPa preferentially partitions Fe into silicate perovskite and reduces its Fe3+ content. The change in oxidation state and partitioning behavior of Fe will increase thermal conductivity and probably could induce a thermal boundary layer at this depth.