Although oxygen is a possible light element in the Earth's core, the effect of pressure on the concentration of this element in metallic iron has been a controversial issue for the last 20 yr. Completely opposite pressure effects have been advocated based on studies of phase relations and element partitioning, respectively. Here we report new data on the partitioning of FeO between magnesiowüstite and liquid iron over a wide pressure-temperature range (3-25 GPa and 2273-3200 K). The proportion of FeO partitioning into liquid iron decreases with increasing pressure below 10 GPa but increases with increasing pressure above 10-15 GPa. The change in the pressure effect is caused by the Fe + O component having different compressibilities in magnesiowüstite and liquid Fe respectively. The new experimental data, together with results from previous studies obtained over a wide P-T range (2-139 GPa, 2273-3150 K), have been fitted by a thermodynamic model that enables the results to be extrapolated to conditions of the outer core. Assuming core-mantle equilibrium, the results show that the outer core is undersaturated in oxygen, which causes a thin layer at the very base of the mantle to be strongly depleted in FeO. The results of core formation modeling indicate that oxygen is likely to be the main light element in the Earth's core (e.g. 7-8 wt.%) and that the FeO content of the proto-earth may have been similar to that of present day Mars (e.g. 18 wt.% FeO in the mantle).
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