Seismic parameters of hcp‐Fe alloyed with Ni and Si in the Earth's inner core

Abstract

Iron alloyed with Ni and Si has been suggested to be a major component of the Earth's inner core. High‐pressure results on the combined alloying effects of Ni and Si on seismic parameters of iron are thus essential for establishing satisfactory geophysical and geochemical models of the region. Here we have investigated the compressional (VP) and shear (Vs) wave velocity‐density (ρ) relations, Poisson's ratio (ν), and seismic heterogeneity ratios (dlnρ/dlnVP, dlnρ/dlnVS, and dlnVP/dlnVS) of hcp‐Fe and hcp‐Fe86.8Ni8.6Si4.6 alloy up to 206 GPa and 136 GPa, respectively, using multiple complementary techniques. Compared with the literature velocity values for hcp‐Fe and Fe‐Ni‐Si alloys, our results show that the combined addition of 9.0 wt % Ni and 2.3 wt % Si slightly increases the VP but significantly decreases the VS of hcp‐Fe at a given density relevant to the inner core. Such distinct alloying effects on velocities of hcp‐Fe produce a high ν of about 0.40 for the alloy at inner core densities, which is approximately 20% higher than that for hcp‐Fe. Analysis of the literature high P‐T results on VP and VS of Fe alloyed with light elements shows that high temperature can further enhance the ν of hcp‐Fe alloyed with Ni and Si. Most significantly, the derived seismic heterogeneity ratios of this hcp alloy present a better match with global seismic observations. Our results provide a multifactored geophysical constraint on the compositional model of the inner core which is consistent with silicon being a major light element alloyed with Fe and 5 wt % Ni.

Shock Compression and Melting of an Fe‐Ni‐Si Alloy: Implications for the Temperature Profile of the Earth's Core and the Heat Flux Across the Core‐Mantle Boundary

“…Further, the observed small isotopic differences between terrestrial samples and meteorites (Georg et al, ) are consistent with silica incorporation during the Earth's core formation. Si as a light element is also supported by recent measurements of density and sound velocity under core conditions, which are generally consistent with seismological data (Fischer et al, ; Lin, Campbell, et al, ; Lin, Struzhkin, et al, ; Liu et al, , ; Mao et al, ; Zhang, Sekine, et al, ). Therefore, knowing the melting temperatures in the Fe‐Ni‐Si system at ICB pressures is critical for determining the thermal structure of the outer core.…”

Sound velocities and density measurements of solid hcp-Fe and hcp-Fe–Si (9 wt.%) alloy at high pressure: Constraints on the Si abundance in the Earth's inner core

“…Diffraction data are also used to detect any 132 developed texture. Examples of collected diffraction patterns are shown in Figure 2 and 133 relationship is also in general agreement with results of ab initio calculations at 0 K. We also 171 noticed the good agreement between the slope of our V S -ρ trend with that obtained by the 172 most recent nuclear resonant inelastic x-ray scattering (NRIXS) experiments [Murphy et al,173 2013; Gleason et al, 2013;Liu et al, 2016], even if actual V S values derived by NRIXS are 174 somewhat lower, between 3 to 8% depending upon datasets. Such a difference is at least 175 partially due to the enrichment in heavier Fe isotopes of samples used for NRIXS studies with 176 respect to sample of natural isotopic abundance used here.…”

High pressure thermoelasticity and sound velocities of Fe-Ni-Si alloys

“…Recently, Sakairi et al (2018) investigated the compressional wave velocity of hcp-Fe 0.89 Si 0.11 up to 1800 K and 84 GPa with inelastic x-ray scattering (IXS), better constraining effect of silicon on compressional velocity at higher temperatures. Fewer studies have investigated the sound velocities of iron-nickel-silicon alloys at 300 K: (Antonangeli et al, 2010) determined the compressional wave velocity from phonon dispersion measurements and (Liu et al, 2016) constrained the shear wave velocities from NRIXS measurements. Most constraints on iron-silicon sound velocities at inner core conditions come from ab initio studies (Vočadlo, 2007;Tsuchiya and Fujibuchi, 2009;Ono, 2013;Martorell et al, 2016).…”

“…Therefore, the plotted v D , v P and v S values for bcc-Fe from this study and our re-analysis of hcp-Fe from Murphy et al (2013) have been corrected to natural enrichment via equation 10. We have also corrected the reported v D , v P , and v S from Lin et al (2005), Liu et al (2016), Mao et al (2001), and Gleason et al (2013) to natural isotopic enrichment using equation 10. As Mao et al (2008) Mao et al (2008) at 50 GPa using helium as a pressure medium.…”

“…The sound velocities of hcp-iron have been extensively studied through shockwave experiments (Jeanloz, 1979;Brown and Mcqueen, 1986;Nguyen and Holmes, 2004), 300 K static compression experiments (Lübbers et al, 2000;Mao et al, 2001;Giefers et al, 2002;Antonangeli et al, 2004;Lin et al, 2005;Mao et al, 2008Mao et al, , 2012Murphy et al, 2013;Gleason et al, 2013;Ohtani et al, 2013;Decremps et al, 2014;Liu et al, 2014Liu et al, , 2016Antonangeli et al, 2018), high temperature static compression experiments (Lin et al, 2005;Sakamaki et al, 2016;Mori et al, 2017), and ab initio studies (Laio et al, 2000;Mao et al, 2001;Steinle-Neumann et al, 2001;Vočadlo et al, 2009;Martorell et al, 2013a;Niu et al, 2015). Similarly, numerous experimental and ab initio studies have investigated hcp-iron's isothermal equation of state (e.g.…”