Thermodynamics of melting relations in the system Fe-FeO at high pressure: Implications for oxygen in the Earth's core

Abstract

The thermodynamics of melting relations in the system Fe-FeO was investigated to the outer core-inner core boundary condition from a self-consistent thermodynamic database which was evaluated from the latest static high-pressure (P) and high-temperature (T) experiments. The evaluated database together with an existing nonideal mixing model for liquids reproduces experimental data on the eutectic composition and temperature to P=50GPa. On the other hand at the outer core pressures (136 to 330GPa), employing an ideal solution model gives calculated eutectic temperatures of T=2990-4330K, which are also consistent with experimental data. Hence, the ideal solution model is applied to calculate the liquid property under outer core conditions and yields the eutectic compositions of Fe-7.2-9.1wt % O. From the Gibbs free energy for the Fe-FeO liquids, I calculated the density, sound velocity, and isentropic temperature gradient of a hypothetical oxygen-bearing outer core. Under the outer core conditions, the addition of oxygen reduces the compressional wave velocity of iron liquid, moving it away from seismologically constrained values. An overall O-rich bulk outer core model is thus excluded. Seismological observations however suggest the presence of a low-velocity layer with a thickness of 60-70km at the top of the outer core. The origin of such a low-velocity layer can be explained by an enrichment of oxygen which might be a consequence of chemical interactions between the core and mantle. ©2014. American Geophysical Union. All Rights Reserved.