Partitioning of Oxygen between Magnesiowüstite and Liquid Iron: Consequences for the Early Differentiation of Terrestrial Planets

Introduction: The partitioning of oxygen between silicates and liquid Fe-rich metallic alloy is a key factor in controlling chemical differentiation during core formation in terrestrial planets [1]. This is because oxygen partitioning determines the oxygen fugacity uniquely for a given bulk composition and therefore controls the partitioning of Fe and other siderophile elements during core formation. Although oxygen was proposed many years ago to be a possible light element in the Earth’s core, the effect of pressure on oxygen solubility in liquid iron has been controversial. Based on phase relations in the FeFeO system, oxygen solubility in liquid Fe increases with increasing pressure. However, according to previous results on the partitioning of oxygen between magnesiowüstite and liquid iron, oxygen solubility in liquid iron has been shown to decrease with increasing pressure [1,2]. Here we report new data on oxygen partitioning between magnesiowüstite and liquid iron in the Fe-FeO-MgO system, obtained over a wide P-T range (3-25 GPa, 2273-3173 K), which resolve this issue. Experimental procedure: Experiments were performed at high pressures and temperatures using a Kawai-type multi-anvil apparatus. Starting materials were mixtures of Fe metal and Fe2O3 oxide powders with a range of bulk oxygen contents. We used cylindrical capsules machined from MgO single crystals as sample containers. Chemical analysis and image analysis of recovered samples were carried out using electron microprobe analysis and scanning electron microscopy respectively. Results: Oxygen contents in the quenched liquid Fe range up to 7.5 wt%. In order to analyze the data, we consider the following reaction of oxygen between magnesiowüstite and Fe liquid: FeO magnesiowüstite = Fe metal + O metal (1). The distribution coefficient, Kd, for this reaction is independent of oxygen fugacity and is given by Kd = XOXFe/XFeO where XO, XFe, XFeO are the mol fractions of oxygen in metal, Fe in metal and FeO in magnesiowüstite, respectively. LnKd increases with increasing temperature over the range of experimental conditions and shows a linear relationship with 1/T at 1524.5 GPa, and 2273-3173 K. The effect of pressure on lnKd is weak up to 25 GPa but lnKd appears to initially decrease with increasing pressure, reach a minimum at 10-15 GPa and then increase at higher pressures (Fig. 1). This behavior is caused by the volume change (ΔV) of reaction (1) being pressure-dependent and changing from being positive at <10 GPa to negative at >15 GPa. The change in ΔV is the result the FeO component being more compressible in liquid Fe than in magnesiowüstite.