Partitioning of Iron Between Liquid and Crystalline Phases of (Mg,Fe)O

Abstract

The density contrast between crystals and coexisting liquids is key to understanding the evolution of the magma ocean. While crystals remain denser than isochemical liquids in most oxide and silicate systems over the pressure range of Earth's mantle, element partitioning, particularly of abundant heavy elements such as iron, can alter the buoyancy. We use molecular dynamics simulations based on spin-polarized density functional theory and adiabatic switching to determine the free energy of iron substitution in liquid and crystalline (Mg,Fe)O and the distribution coefficient. We find that iron is strongly incompatible with a distribution coefficient less than 0.3 over the mantle pressure range. We find that the crystal is buoyant with respect to coexisting liquid at pressures exceeding 50 GPa. The high-spin to low-spin transition has an important influence on the distribution coefficient, which decreases with increasing pressure up to 80 GPa, and then increases on further increase of pressure.