Closure of the Fe-S-Si liquid miscibility gap at high pressure

A high pressure investigation of melting relationships in the Fe-S-Si system has been conducted in a multi-anvil apparatus from 10 to 27 GPa and up to 2343 K. At 1 atm, the Fe-S-Si ternary system exhibits a vast miscibility gap [Raghavan, V., 1988. Phase diagrams of ternary iron alloys. Part 2: Ternary systems containing iron and sulphur. Indian Institute of Metals, Calcutta]. Quenched samples from experiments conducted at 10 and 12 GPa show an emulsion of immiscible liquids (an Fe-S melt and an Fe-Si melt). The liquid miscibility gap persists to at least 2343 K at 10 GPa. At 15 GPa, only one liquid is quenched, with a fine homogeneous dendritic texture. The results provide a mechanism to incorporate both S and Si as the light elements into the Earth’s core during a moderately high-pressure differentiation, consistent with geochemical models predicting up to 15 wt.% of light elements in the Earth’s core with 2-5 wt.% S and 7-10 wt.% Si. In contrast, for small planets such as Mars and Ganymede, differentiation took place within the pressure range of the miscibility gap. The composition of these cores is likely to be S-rich but Si-poor.