Oxygen Partitioning Between Magnesiowustite And Liquid Fe Rich Metal At High Pressure And High Temperature

Oxygen is one of the possible candidates for being the light element in the Earth's core. The pressure effect on oxygen solubility in metallic iron has been controversial. From phase relations in the Fe-FeO system, it has been suggested that oxygen solubility in liquid iron increases with increasing pressure. However, from partitioning studies between magnesiowustite and liquid metal, it has been proposed that oxygen solubility in liquid iron decreases with increasing pressure. Here we report new data on oxygen partitioning between magnesiowustite and liquid iron in the Fe-FeO-MgO system over a wide pressure and temperature range (3-25 GPa and 2273-3173 K), which can resolve this inconsistency. We calculate the distribution coefficient, Kd, for the partitioning of oxygen between magnesiowustite and liquid Fe: Kd = XmetOXmetFe/XmwFeO. We have examined the present experimental data by applying the following equation: RTlnKd = -ΔH + TΔS - ∫ΔVdP. We observed that lnKd decreases with increasing pressure up to approximately 10 GPa, but reaches a minimum between 10-15 GPa. This means that the volume change of the reaction (ΔV) is strongly pressure dependent being negative at pressure below 10 GPa but becoming positive at higher pressures. This can be easily explained by a difference in the compressibility between the FeO components of magnesiowustite and liquid Fe. Our new partitioning data provide a solution to the previous arguments over the pressure effect on oxygen solubility between Fe-FeO phase relation studies and partitioning studies. Oxygen solubility in metallic Fe decreases with increasing pressure at lower pressure conditions but increases with increasing pressure above 15 GPa as observed in studies of phase relations. The previous partitioning studies overlooked the ΔV change because of the limited pressure conditions examined. The present results suggest that the FeO in a deep terrestrial magma ocean may have been incorporated into Fe metal during core formation. Oxygen may very well be the light element in the core.