Partitioning of silicon and sulfur between solid and liquid iron under core pressures: Constraints on Earth's core composition

Abstract

Not only the Earth's outer core but also the inner core contains light elements, which causes reductions in density and velocity with respect to those of pure solid iron. Silicon, sulfur, and possibly hydrogen are believed to be important inner core light elements, because their partition coefficients between solid Fe and liquid, D (solid/liquid), are high (>0.7) under inner core pressures unlike other possible light elements such as oxygen and carbon. Recent experiments demonstrated that DSi is remarkably enhanced with increasing sulfur concentration in liquid at 15–21 GPa. Here we conducted solid/liquid partitioning experiments on the Fe-Si-S(-C) system up to the Earth's core pressure range of 189 GPa in a laser-heated diamond-anvil cell (DAC) and examined the effect of silicon-sulfur interaction on both DSi and DS. The results show that DS significantly diminishes with increasing the silicon content in liquid, while DSi increases with higher sulfur concentration. Based on these partitioning data, we explored the pairs of possible liquid outer core and solid inner core compositions in the Fe-Si-S±H±O system, which account for their respective seismological observations of the density and velocities. For the temperature at the inner core boundary of 5400 K, we obtained Fe + Ni + 2.7–3.0 wt% Si + 1.7 wt% S (fixed) + 0.32–0.59 wt% H + 0.05–2.5 wt% O for the outer core and Fe + Ni + 2.5–2.8 wt% Si + 0.69–0.70 wt% S + 0.23–0.43 wt% H for the inner core.