Partitioning of FeO between liquid Fe-Ni metal and magnesiowüstite at high pressures and temperatures

The Earth's outer core is believed to be composed of liquid Fe-Ni alloy together with one or more light elements of which oxygen is a possible candidate. Systematic investigations of the partitioning of oxygen between Fe-Ni liquid and silicates/oxides at high pressures and temperatures are scarce, although such studies have been performed on partitioning between Fe melt and silicates/oxides up to core-mantle boundary pressures (e.g. Takafuji et al. 2005; Sakai et al. 2006; Asahara et al. 2007). Their experimental results and thermodynamic interpretations show that the partitioning of FeO into Fe melt, in equilibrium with silicates/oxides, has a minimum value at a pressure of 15-20 GPa (Asahara et al. 2007). O'Neill et al. (1998) and Rubie et al. (2004) showed that the partitioning of FeO into Fe-Ni melt in equilibrium with magnesiowüstite decreases in the pressure range of 5- 23 GPa. In this study, we have investigated the partition coefficient of FeO between Fe-Ni melt and magnesiowüstite (Kd=XmetFeXmetO/XmvFeO) at 5-24.5 GPa and 2473-2873 K in order to clarify the effect of Ni on the partitioning of FeO into Fe liquid. High-pressure experiments were performed using a multianvil apparatus at the Bayerisches Geoinstitut. Starting metallic compositions were Fe-10 wt.% Ni and Ni-free Fe to which 0.5-1.5 wt.% O were added. Both compositions were run simultaneously in each experiment by containing them in a double-chamber MgO capsule. Our experimental results show that the Kd for Fe-10 wt.% Ni melt + magnesiowüstite is lower than for Fe melt + magnesiowüstite. The value of Kd for Fe-Ni melt + magnesiowüstite increases with temperature, but the pressure dependence is not clearly defined by our results. Based on this study, the concentration of oxygen in the Earth's core could be lower than previously estimated, although the effect of sulfur on oxygen solubility still needs to be investigated systematically.