Melting experiments on Fe-S-O-C alloys at Martian core conditions: Possible structures in the O- and C-bearing core of Mars

Recent seismological studies of the Martian core revealed its relatively low density, suggesting the presence of large amounts of light elements including oxygen and carbon in addition to sulfur. In order to reveal crystallizing solids in the light-element-rich core of Mars, we performed high-pressure melting experiments on Fe-S-O-C alloys at 26-49 GPa using a laser-heated diamond-anvil cell. The liquidus phase relations in the Fe-S-O-C system were determined based on textural and chemical characterizations of recovered samples. The results show that Fe-S-OC liquids crystallize FeO or Fe3C in the presence of small amounts of O or C in liquids. Accordingly the liquidus fields of Fe3S and Fe2S are small, and the quaternary eutectic point is found to be close to the Fe-Fe3S binary eutectic point. Under Martian core conditions, S-rich liquids have low liquidus temperatures to crystallize FeO or Fe3C compared to S-poor liquids. The pressure dependence of liquidus temperatures suggests that crystallization of Mars' core starts at the center upon cooling. According to the FeO-Fe3C cotectic surfaces and their liquidus temperatures, an FeO and/or Fe3C inner core is predicted unless the Martian core remains entirely liquid.