Partitioning of Oxygen Between Ferropericlase and Earth's Liquid Core

Abstract

Transfer of oxygen between Earth's core and lowermost mantle is important for determining the chemistry and nature of stratification on both sides of the core-mantle boundary (CMB). Previous studies have found that oxygen enters the metal when Fe-O liquid equilibrates with representative lower mantle materials. However, experiments have not yet been conducted at CMB pressure-temperature conditions. Here we use density functional theory to obtain the first estimates of oxygen partitioning between liquid Fe-O-Si metals and ferropericlase at CMB conditions. Our method successfully reproduces experimentally derived partitioning data at 134 GPa and 3200 K, while our calculations show a strong increase of oxygen partitioning into metal with temperature and a weaker increase with pressure, consistent with previous work. At CMB conditions of 135 GPa and 4000–4700 K oxygen partitioning into metal is higher than previous estimates and increases strongly with metal oxygen concentration. Analysis of the lower mantle chemical boundary layer shows that oxygen transport through the solid is severely limited even with the enhanced partitioning and is unlikely to explain the thickness of a stably stratified layer below the CMB inferred from seismology. However, if the lower mantle was molten in early times, as suggested by core evolution models with high thermal conductivity, then the mass flux and stable layer thickness are significantly increased.