Immiscible two-liquid regions in the Fe–O–S system at high pressure: Implications for planetary cores

We have determined phase relations in the Fe–O and Fe–O–S systems in the range of 15–21 GPa and 1825–2300 °C. Below the liquidus temperatures, solid FeO and metallic liquids are observed in both the Fe–O and the Fe–O–S systems. An immiscible two-liquid region exists in the Fe–O binary system in the pressure range investigated, and the immiscibility gap between Fe-rich metallic liquid and FeO-rich ionic liquid does not greatly change with either pressure or temperature. On the other hand, an immiscible two-liquid region in the Fe–O–S ternary system narrows significantly with increasing pressure at constant temperature and vice versa, and it almost disappears at 21 GPa, and 2300 °C. Immiscible two-liquid regions are thus not expected to exist in the Fe–O–S system in the Earthʹs core, suggesting that both oxygen and sulfur can be incorporated into the core. Our results are consistent with a geochemical model for the core containing 5.8 wt.% oxygen and 1.9 wt.% sulfur as proposed by McDonough and Sun [McDonough, W.F., Sun, S.-S., 1995. The composition of the Earth. Chem. Geol. 120, 223–253].