Strong compositional gradient in the Earth's inner core?

Abstract

Dynamic motions in the Earth's solid inner core driven by thermo-chemical buoyancy, such as plume convection and translation, have been proposed to explain seismic observations. The inner core should be chemically homogeneous if it is actively agitated. However, its high thermal conductivity may suppress such motions. Here we computed the equations of state for six hundred and ninety-three different Fe-Ni-Si-S-H alloys and compared their density (ρ) and bulk sound velocity (V_Φ) profiles with inner core seismic reference models. While such calculations were made at static conditions, we additionally calculated the Helmholtz energy by using the quasi-harmonic approximation to obtain the ρ and V_Φ of hexagonal close-packed (hcp) alloys under high temperatures relevant to the inner core. The results demonstrate that the changes in ρ of these hcp alloys along the inner core P-T profile are comparable to the radial ρ gradients shown by the PREM and AK135 models, but none of the Fe-Ni-Si-S-H alloys explain the gentle gradients in V_Φ of these reference models. Given that the PREM and AK135 models provide the correct V_Φ gradient, it suggests compositional stratification in the solid inner core, much stronger than can be developed upon crystallization from a homogeneous liquid outer core. The inner core might have crystallized from a chemically stratified liquid core (S-poor and H-rich toward the centre), which possibly formed as a result of liquid immiscibility between S-rich and H-rich liquids.