Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Compressional wave velocity-density (V(P)--ρ) relations of candidate Fe alloys at relevant pressure-temperature conditions of the Earth's core are critically needed to evaluate the composition, seismic signatures, and geodynamics of the planet's remotest region. Specifically, comparison between seismic V(P)--ρ profiles of the core and candidate Fe alloys provides first-order information on the amount and type of potential light elements--including H, C, O, Si, and/or S-needed to compensate the density deficit of the core. To address this issue, here we have surveyed and analyzed the literature results in conjunction with newly measured V(P)--ρ results of hexagonal closest-packed (hcp) Fe and hcp-Fe(0.85)Si(0.15) alloy using in situ high-energy resolution inelastic X-ray scattering and X-ray diffraction. The nature of the Fe-Si alloy where Si is readily soluble in Fe represents an ideal solid-solution case to better understand the light-element alloying effects. Our results show that high temperature significantly decreases the V(P) of hcp-Fe at high pressures, and the Fe-Si alloy exhibits similar high-pressure V(P)--ρ behavior to hcp-Fe via a constant density offset. These V(P)--ρ data at a given temperature can be better described by an empirical power-law function with a concave behavior at higher densities than with a linear approximation. Our new datasets, together with literature results, allow us to build new V(P)--ρ models of Fe alloys in order to determine the chemical composition of the core. Our models show that the V(P)--ρ profile of Fe with 8 wt % Si at 6,000 K matches well with the Preliminary Reference Earth Model of the inner core.