Phase relationships of the Fe–FeS system in conditions up to the Earth's outer core

Abstract

In situ X-ray diffraction experiments in the Fe–FeS system were performed up to 220 GPa and 3300 K using a laser-heated diamond anvil cell. Fe 3S and ε-Fe coexisted stably up to 220 GPa and 3300 K, and thus, Fe 3S is likely to be the stable S-bearing iron alloy under the Earth's core conditions. The solid iron ( ε-Fe) also contained 7.6(0.8) at.% of sulfur at 86 GPa and 2200 K. The amount of sulfur in the solid iron increased with increasing pressure at the eutectic temperatures. If the sulfur content obtained in this study is extrapolated to the conditions at the inner core, all the sulfur in the solid inner core can be stored in ε-Fe. The eutectic composition becomes nonsensitive to pressure and seems to be constant around 20 at.% of sulfur at pressures above 40 GPa. The pressure gradient of the melting curve of the Fe–FeS system is 13.4(0.7) K/GPa. Based on our results of melting relationship, the temperature at the core–mantle boundary should be greater than 2850(100) K, assuming that sulfur is the only light element in the Earth's liquid outer core.