The effects of nickel and sulphur on the core–mantle partitioning of oxygen in Earth and Mars

Abstract

Constraints on the partitioning of oxygen between silicates, oxides, and metallic liquids are important for determining the amount of oxygen that may have entered the cores of terrestrial planets and to identify likely reactions at the core–mantle boundary. Several previous studies have examined oxygen partitioning between liquid Fe metal and ferropericlase, however, the cores of terrestrial planets also contain nickel and most likely sulphur. We have performed experiments to examine the effects of both nickel and sulphur on the partitioning of oxygen between ferropericlase and liquid Fe alloy up to pressures of 24.5 GPa in the temperature range 2430–2750 K using a multianvil press. The results show that at a fixed oxygen fugacity the proportion of oxygen that partitions into liquid metal will decrease by approximately 1–2 mol% on the addition of 10–20 mol% nickel to the liquid. The addition of around 30 mol% sulphur will, on the other hand, increase the metal oxygen content by approximately 10 mol%. Experiments to examine the combined effects of both nickel and sulphur, show a decrease in the effect of nickel on oxygen partitioning as the sulphur content of the metal increases. We expand an existing thermodynamic model for the partitioning of oxygen at high pressures and temperatures to include the effects of nickel and sulphur by fitting these experimental data, with further constraints provided by existing phase equilibria studies at similar conditions in the Fe–S and Fe–O–S systems. Plausible terrestrial core sulphur contents have little effect on oxygen partitioning. When our model is extrapolated to conditions of the present day terrestrial core–mantle boundary, the presence of nickel is found to lower the oxygen content of the outer core that is in equilibrium with the expected mantle ferropericlase FeO content, by approximately 1 weight %, in comparison to nickel free calculations. In agreement with nickel-free experiments, this implies that the Earth's outer core is undersaturated in oxygen with respect to plausible mantle FeO contents, which will result in either the depletion of FeO from the base of the mantle or cause the development of an outer core layer that is enriched in oxygen. The oxygen content of the more sulphur-rich Martian core would be in the range 2–4 wt.% if it is in equilibrium with the FeO-rich Martian mantle.