Effect of Ni on Fe–FeS phase relations at high pressure and high temperature

A series of melting experiments in the Fe-rich portion of the Fe–Ni–S system have been conducted at 19–23 GPa and 800–1100 °C. The solubility of S in the Fe–Ni solid alloy and the eutectic melting in the Fe–Ni–S system were determined as a function of Ni content. The maximum S solubility in the Fe–Ni alloy is 2.7 wt.% at 20 GPa and the eutectic temperature. The eutectic melting temperature in the Fe–Ni(5wt.%)–S system is ~ 1000 °C lower than the melting point of pure Fe at 20 GPa. We also found that Ni can substitute Fe in the Fe3S structure to form (Fe,Ni)3S solid solutions up to at least a Fe/Ni atomic ratio of 0.5. Similar to melting behavior in the Fe–FeS system, the eutectic melting relations in the Fe–Ni–S system could produce inner and outer cores with the right light element balance to account for the density difference between the solid inner core and the liquid outer core.