Chemical equilibrium in the Earth's core*

The core of the Earth is a source of thermal energy for the mantle, helping to drive convection, plate tectonics and volcanism. It is mainly formed by iron, but is also contains light impurities. The exact chemical inventory of the core is unknown, but it is believed that oxygen may be present in relatively large quantities, as it is a major element in the mantle. Freezing of the inner core causes oxygen to be released in the liquid, which is thought to be the main form of energy driving core convection at the present day, responsible for the generation of the magnetic field. One of the fundamental questions is therefore how oxygen entered the core in the first place.
Using first principles calculations of chemical potentials we put contraints on the equilibrium concentrations of oxygen between liquid iron and a liquid silicate mixture, representative of long lived magma ocean (MO) at the base of the mantle. We show that the presence of a large fraction of oxygen in the core can be explained by a relatively large thermodynamic advantage of partitioning from the MO into the liquid core.
We also computed chemical potentials in solid ferropericlase, thought to be one of the main constituents of the Earth's mantle, and found that the current oxygen concentration in the core is lower than its equilibrium concentration, suggesting that the mantle may be continually pumping oxygen into the core, even at the present day. This has important consequences for our understanding of convection in the core, supporting the idea of the presence of a stratified, oxygen rich, layer at the top of the core, which may have been observed in the seismological record.