Strong compositional gradient in the Earth's inner core?

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. Our first-principles calculations of 693 Fe-Ni-Si-S-H alloys (blue lines) demonstrate that the changes in density ( ρ ) of these hcp alloys along the inner core P-T profile are compatible to the radial ρ gradient by the PREM ( red lines in a , c ), but none of the Fe-Ni-Si-S-H alloys do not explain the gentle gradient in V Φ ( b , d ). • We computed the equations of state for 693 different hcp Fe-Ni-Si-S-H alloys. • The PREM V Φ gradient in the inner core is not explained by a single composition. • The inner core might have crystallized from a chemically stratified liquid core.