Melting relations in the Fe-S-Si system at high pressure and temperature: Implications for the thermal structure of the planetary cores

It is widely accepted that the Earth's core is mainly composed of iron and contains light elements to account for its density deficit. Alloying with light elements significantly affects the physical properties of iron and depresses its melting temperature. Therefore, the melting relation of the Fe-light elements system is the key to clarify the thermal structure of the Earth's core. Although there are many candidates for light elements in the core, sulfur and silicon are considered to be the major light elements. Some geochemical models predicted that sulfur and silicon could be present not only in the core of the Earth but also in the core of other terrestrial planets such as Mars and Mercury. To better understand the properties of the planetary cores, we investigated the melting relations of the Fe-S-Si system under high-pressure conditions. Here, we report the melting relations in the Fe-S-Si system up to 60 GPa. Melting experiments were performed in the pressure range of 20-60 GPa and the temperature range of 1300-2500 K using a double-sided laser-heated diamond anvil cell combined with X-ray diffraction technique. In situ X-ray diffraction experiments were conducted at the BL10XU beamline of the SPring-8 facility. The melting detection was based on disappearance of the X-ray diffraction peaks of the sample. On the basis of X-ray diffraction patterns, we confirmed that iron-silicon alloy which hcp and fcc structure and Fe3S are stable phases under subsolidus conditions. Both solidus and liquidus temperatures are significantly lower than the melting temperature of pure Fe and increases with pressure in this study. In order to draw the melting curve as a function of pressure, we fitted the present results using the Simon's equation. Our results could provide important constraints on the thermal structure of the planetary cores.