Seismic parameters of hcp-Fe alloyed with Ni and Si in the Earth's inner core

Iron alloyed with Ni and Si has been suggested to be a major component of the Earth’s inner core. High-pressure results on the combined alloying effects of Ni and Si on seismic parameters of iron are thus essential for establishing satisfactory geophysical and geochemical models of the region. Here we have investigated the compressional (VP) and shear (Vs) wave velocity-density (ρ) relations, Poisson’s ratio (ν), and seismic heterogeneity ratios (dlnρ/dlnVP, dlnρ/dlnVS, and dlnVP/dlnVS) of hcp-Fe and hcp-Fe86.8Ni8.6Si4.6 alloy up to 206GPa and 136GPa, respectively, using multiple complementary techniques. Compared with the literature velocity values for hcp-Fe and Fe-Ni-Si alloys, our results show that the combined addition of 9.0wt% Ni and 2.3wt% Si slightly increases the VP but significantly decreases the VS of hcp-Fe at a given density relevant to the inner core. Such distinct alloying effects on velocities of hcp-Fe produce a high ν of about 0.40 for the alloy at inner core densities, which is approximately 20% higher than that for hcp-Fe. Analysis of the literature high P-T results on VP and VS of Fe alloyed with light elements shows that high temperature can further enhance the ν of hcp-Fe alloyed with Ni and Si. Most significantly, the derived seismic heterogeneity ratios of this hcp alloy present a better match with global seismic observations. Our results provide a multifactored geophysical constraint on the compositional model of the inner core which is consistent with silicon being a major light element alloyed with Fe and 5wt% Ni.