Understanding the sound velocities and EoS of Fe-Ni-Si alloys in the Earth’s Inner Core through Joint Experimental Measurements

Abstract:

Iron with with 5-10% nickel and a small amount of light element(s) has been suggested to be the major component of the Earth’s core. The lattice preferred orientation of the Fe light element alloy may be responsible for observed seismic V_P and V_S anisotropies in the inner core. The alloying effects of nickel (a heavier element than iron) and silicon (a potential light element) on sound velocities and EoS parameters of Fe are thus of great importance to establish satisfactorily geophysical and geochemical models of the core. Here we have systematically investigated the density-velocity relation and elastic anisotropies of Fe, Fe-Si and Fe-Ni-Si alloys using combined results from in situ high energy resolution inelastic X-ray scattering (HERIX) (sensitive to V_P), nuclear resonant inelastic X-ray scattering (NRIXS) (sensitive to V_D and Vs), and X-ray diffraction (including radial X-ray diffraction) measurements in high-pressure diamond anvil cells at various temperatures. Our results also show that the measured density-velocity (ρ-V_P, -V_D, or -V_S) profiles of Fe, Fe-Si and Fe-Ni-Si alloys can be better fitted using an empirical power-law function, instead of a linear approximation, while a visibly temperature effect on the sound velocities is also observed. Fe and Fe-Si and Fe-Ni-Si alloys exhibits similar high-pressure density-velocity behavior via a constant density offset. With comparison to literature results, nickel seems to have less effect on compressional-wave velocity (V_P) than on shear-wave velocity (V_S), while silicon has a stronger effect on V_P than V_S. The measured apparent V_P anisotropy of hcp-Fe at high pressures is comparable to the inner core V_P anisotropy, which might be enhanced by Si substitution. Together with thermoelastic modeling, our results indicate that the Earth’s inner core may contain about twice more light element(s) such as Si than previously thought, together with the presence of 10 at.% Ni, in order to match with the PREM seismic model.