Hexagonal close‐packed (hcp) structured Fe‐Ni alloy is believed to be the dominant phase in the Earth's inner core. This phase is expected to contain 4%–5% light elements, such as Si and H. While the effects of individual light element candidates on the equation of state (EoS) of the hcp Fe metal have been studied, their combined effects remain largely unexplored. In this study, we report the equations of state for two hcp‐structured Fe‐Si‐H alloys, namely Fe0.83Si0.17H0.07and Fe0.83Si0.17H0.46, using synchrotron X‐ray diffraction measurements up to 125 GPa at 300 K. These alloys were synthesized by cold compression of Fe‐9wt%Si in either pure H2or Ar‐H2mixture medium in diamond‐anvil cells. The volume increase caused by a H atom in hcp Fe‐Si‐H alloys is approximately eight times greater than that by a Si atom. We used the improved data set to develop a composition‐dependent EoS that covers a wide range of compositions. Our calculated density and bulk sound velocity of hcp Fe‐Si‐H alloys suggest a large trade‐off between Si and H contents in fitting the seismic properties of the inner core. Combining our new EoS with geophysical and geochemical constraints, we propose 1.6–3 wt% Si and 0.15–0.6 wt% H in the Earth's inner core.
The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe90Ni10‐3 wt.% C and Fe90Ni10‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe90Ni10liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (
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- DOI PREFIX: 10.1029
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- Journal of Geophysical Research: Solid Earth
- Medium: X
- Sponsoring Org:
- National Science Foundation
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