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

Haijun Huang,Jidong Weng,Feng Xu,Yingwei Fei,Lili Fan,Qingsong Wang,Xun Liu,Chunwei Leng,Xiang Wang,Gang Yang,Lingcang Cai,Ye Wu

doi:10.1038/s41467-022-28255-2

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.

Highlights

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
Measurements by IXS17,18 and nuclear-resonant inelastic X-ray scattering (NRIXS)[19] showed that VP decreases with temperature (
Estimate of the temperature effect at moderate and high temperature may be doubtful because the extraction of the phonen density of state is from the NRIXS spectra based on a quasi-harmonic model[16]

Summary

Introduction

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. Any proposed composition models of the core must simultaneously satisfy the core density (ρ), the bulk sound velocity (VB) of the liquid outer core, and the compressional wave velocity (VP) and shear wave velocity (VS) of the solid inner core, defined by seismic observations[14,15]. High temperature and the presence of certain light elements such as C27,28 in the Earth’s inner core are expected to decrease the VS value and increase the σ value to match the observations. To model the VS profile of the inner core, we need to determine the temperature and compositional dependence of VS of iron alloys under high P-T conditions relevant to the Earth’s inner core. The simulated results considering the effect of anharmonicity[29] revealed that both VP and VS of iron decreased with temperature at constant density and could match the seismic data of the inner core. More recent calculations[30,31] showed that both VP and VS of iron increased almost linearly with density between

Methods

Results

Conclusion