Journal Article

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

Lin, Jung-Fu, Zhao, Jiyong, Mao, Zhu, Liu, Jin, Alatas, Ahmet, Mao, Ho-Kwang, Gao, Lili

2012

Overview

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 ₀.₈₅Si 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.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: Alloys / chemical composition / data collection / Density / Diffraction / Earth / Earth core / geophysics / High pressure / High temperature / Inner cores / iron / Light elements / Particle velocity / Physical Sciences / Planetary cores / Pressure / Scattering / silicon / Solid solutions / temperature / Wave velocity / X-radiation / X-ray diffraction