Pressure–temperature cartography of Fe–S–Si immiscible system

Multi-anvil press experiments were performed using a single cell assembly containing six different compositions. This set-up allows a careful sampling of the miscibility gap for given P–T conditions. Shrinking of the miscibility gap in the Fe–S–Si system has been studied from 4 to 12 GPa up to 2200 K, demonstrating a stable immiscible zone up to 4 GPa and 2200 K and its closure at higher pressures. Presence of both S and Si in the Earth’s core is suggested by chondritic models. Therefore, its composition is inherited from processes at pressures higher than 4 GPa. This evolution of the Fe–S–Si miscibility gap is linked with the change in the local short-range order in Fe and Fe–S liquids. Our results indicate that core formation under reducing conditions would be affected by immiscibility for planetesimals up to size of the Moon. Furthermore, due to the difference in wetting properties between the two immiscible liquid phases, the S-rich metal phase would control the chemical exchange between liquid metals and silicates during early differentiation in planetesimals.