Inner core composition paradox revealed by sound velocities of Fe and Fe-Si alloy

Abstract

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (VS) and high Poisson’s ratio (σ) in the solid inner core. To date, neither VS nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present VS and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low VS and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease VS and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.

High Pressure Melting Curve of Fe Determined by Inter‐Metallic Fast Diffusion Technique

“…Efforts have been made both theoretically (e.g., Alfè et al., 2002; Vočadlo et al., 2003) and experimentally ‐ by dynamic (Brown & McQueen, 1986; Huang et al., 2022; Nguyen and Holmes, 2004; Turneaure et al., 2020; Yoo et al., 1993) or static compression (e.g., Anzellini et al., 2013; Aquilanti et al., 2015; Hou et al., 2021; Jackson et al., 2013; Shen et al., 2004; Sinmyo et al., 2019; Strong, 1959; Williams et al., 1987; Yong et al., 2019; Zhang et al., 2016) to study the melting curve of Fe at high‐pressure. The static compression experiments using laser‐heating diamond anvil cell (DAC) have been focused on melting measurements at pressures above 25 GPa, with application to the Earth's core.…”

Investigating metallic cores using experiments on the physical properties of liquid iron alloys

“…The bulk sound velocity of the Fe-H liquid would be about 2% higher than the room-temperature values based on the values recently determined for Fe and Fe-Si alloy (Huang et al, 2022). Finally, the dataset for Fe-P alloys is currently limited to one study that identified a decrease in Vp of ~0.5 km/s when pressure increases from 40 to 60 GPa and temperature from 2,450 to 2,800 K (Kinoshita et al, 2020).…”

Unveiling the outer core composition with neutrino oscillation tomography

“…As mentioned in the introduction, silicon and oxygen are not the only light elements that have been proposed in conventional models of the outer core; also sulphur and carbon could be present (Poirier, 1994), but their Z/A is too close to that of Si and O to allow for a discrimination by means of this method. Furthemore, high-pressure/high-temperature thermodynamics modelling appear to rule out Fe-Si-C compositions (Huang et al, 2022). Therefore we do not consider them further in this study.…”