Sound Velocity Measurements of B2‐Fe‐Ni‐Si Alloy Under High Pressure by Inelastic X‐Ray Scattering: Implications for the Composition of Earth's Core

Abstract

Our knowledge of the structure and composition of Earth's core is based on sparse direct evidence (e.g., from seismology, geodesy, geo-and paleo-magnetism) and many indirect observations (from cosmochemistry, experimental petrology, and mineral physics) (Allègre et al., 1995;McDonough & Sun, 1995). Cosmochemical studies on iron meteorites and a comparison of mineral physics data with seismological observations (measurements of density (ρ) and compressional (V P ) and shear (V S ) wave velocities under extreme conditions) suggest that Earth's inner core is primarily composed of metallic Fe-Ni alloy (5-25 wt.% Ni) (McDonough & Sun, 1995;Wasson & Chou, 1974). However, the inner core density is ∼5 % lower than pure Fe at corresponding pressures and temperatures (Dewaele et al., 2006;Fei et al., 2016), presumably due to the presence of light elements (Birch, 1952) that were incorporated in the core during its formation (

Thermoelastic Properties of B2‐Type FeSi Under Deep Earth Conditions: Implications for the Compositions of the Ultralow‐Velocity Zones and the Inner Core

“…Our V P and V S results align well with previous experimental results at ∼110 GPa and 1,600 K from nuclear inelastic scattering measurements (Mergner et al, 2021), and calculation results at ∼109 GPa and 315-334 GPa from first-principles molecular dynamics simulations by Vočadlo (2007), but the our V S results are significantly lower than simulation results reported in Ono (2013). Recent experiments combining high-resolution inelastic X-ray scattering and power X-ray diffraction in diamond anvil cells, conducted up to 100 GPa and at room temperature (Dominijanni et al, 2022), suggested lower V P and V S values but higher density for B2-Fe 0.67 Ni 0.06 Si 0.27 compared to our results. These differences are likely due to the difference in compositions.…”

“…The gray dashed vertical lines indicate pressures at the core‐mantle boundary (CMB) and inner‐core boundary (ICB). The circle, square, triangle, and diamond markers represent results from M2021 (Mergner et al., 2021), V2007 (Vočadlo, 2007), D2022 (Dominijanni et al., 2022), and O2013 (Ono, 2013), respectively. Horizontal red, green, and blue lines represent PREM values and velocity anomalies (−10% and −20% for V P and −15% and −30% for V S ) for density, V P , and V S , respectively.…”

Sound velocity of hexagonal close-packed iron to the Earth’s inner core pressure

“…Therefore, our results are more definite compared to the previous estimations. Additionally, previous studies provided contradictory estimates in silicon and sulphur contents in the inner core, ~5‒8 wt% 34 , 35 and ~1‒4 wt% 10 , 32 , 33 in silicon content, whereas the sulphur content varies from 1.3 to 3.7 wt% 36 , 37 . The present analysis suggests that both silicon and sulphur may be needed to account for the properties of the inner core.…”

“…Our estimations of the silicon and sulphur contents of the inner core are based on the deficits of compressional and shear velocities and density, whereas most previous estimations were based only on the density deficit of the inner core, and occasionally, the v p deficit 10 , 32 , 33 . Therefore, our results are more definite compared to the previous estimations.…”

“…The relation between the excitation energy, v p and the momentum transfer is given by where E is excitation energy, h is Plank’s constant, Q is momentum transfer, and Q max is the averaged distance to the edge of the first Brillouin zone, including the effect of the preferred orientation 55 , 60 . To obtain the v p from the IXS spectra, the dispersion was fit with the above sine equation 5 – 10 , 16 , 25 , 29 , 33 , 46 , 47 , 50 , 55 , 57 . A weighted least-squares method was used with v p and Q max as free parameters.…”