Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Abstract

Compressional wave velocity-density (V P − ρ) relations of candidate Fe alloys at relevant pressure-temperature conditions of the Earth's core are critically needed to evaluate the composition, seismic signatures, and geodynamics of the planet's remotest region. Specifically, comparison between seismic V P − ρ profiles of the core and candidate Fe alloys provides first-order information on the amount and type of potential light elements-including H, C, O, Si, and/or S-needed to compensate the density deficit of the core. To address this issue, here we have surveyed and analyzed the literature results in conjunction with newly measured V P − ρ results of hexagonal closest-packed (hcp) Fe and hcp-Fe 0.85 Si 0.15 alloy using in situ highenergy resolution inelastic X-ray scattering and X-ray diffraction. The nature of the Fe-Si alloy where Si is readily soluble in Fe represents an ideal solid-solution case to better understand the lightelement alloying effects. Our results show that high temperature significantly decreases the V P of hcp-Fe at high pressures, and the Fe-Si alloy exhibits similar high-pressure V P − ρ behavior to hcp-Fe via a constant density offset. These V P − ρ data at a given temperature can be better described by an empirical power-law function with a concave behavior at higher densities than with a linear approximation. Our new datasets, together with literature results, allow us to build new V P − ρ models of Fe alloys in order to determine the chemical composition of the core. Our models show that the V P − ρ profile of Fe with 8 wt % Si at 6,000 K matches well with the Preliminary Reference Earth Model of the inner core.compressional-wave velocity | high pressure-temperature E nigmatic properties of the Earth's inner core have recently been discovered including differential super-rotation (1), seismic anisotropies (2-4), and fine-scale seismic heterogeneities (5, 6). Deciphering these observations requires solid knowledge about the composition of the Earth's inner core and, therefore, the elasticity of candidate Fe alloys (7-16). Since F. Birch pointed out in the 1950s that Earth's core is too dense if composed of Fe or Fe-Ni alloy alone (13), a number of candidate major light elements, including oxygen (O), silicon (Si), sulfur (S), carbon (C), and hydrogen (H), have been suggested via cosmochemical, geochemical, and geophysical evidence (17). To ascertain the identity and exact amount of light elements needed in the Earth's inner core, one key piece of information lies in the comparison of the seismic V P − ρ profiles with reliable laboratory measurements of these properties for candidate Fe alloys. Potential Fe-light element alloy must have V P − ρ profiles that match seismic models such as the Preliminary Reference Earth Model and AK135 (7,8). Thus, this requires precise experimental results describing the V P − ρ relationships of Fe alloys at pressure-temperature (P-T) conditions relevant to the Earth's core. To address this issue, here, we present new experimental measurements on the V...

High-pressure studies with x-rays using diamond anvil cells

“…Sound velocities of materials at HP to over 1 Mbar have been measured [12,[285][286][287] using HERIX. With single-crystal samples, complete phonon dispersions in the Brillouin zone can be mapped by HERIX at HP [100,288,289].…”

“…From the low-energy region of each phonon DOS, Debye sound velocity can be derived. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Phonon DOS has been determined for a number of iron compounds and alloys: FeO [519,520], FeS [521], Fe-Ni [522], Fe-Si alloys [287,522], FeH [523], Fe3S [524], Fe3C [525][526][527], Fe2O3 [528], (Mg,Fe)O [497], and (Mg,Fe)SiO3 enstatite [529]. For nanocrystalline Fe, large distortions were found in its phonon DOS [530], showing a lifetime broadening at high energies and an enhancement in its phonon DOS at energies below 15 meV.…”

“…Examples of sound velocity determination using NRIXS include: Fe [63,297,517,518], FeO [519,520], FeS [521], Fe-Ni [522], Fe-Si alloys [287,522], FeH [523], Fe3S [524], Fe3C [525][526][527], Fe2O3 [528], and (Mg,Fe)O [497]. The Debye parabola is best constrained at the low-energy limit, and a resolution of better than 2 meV is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 therefore essential in NRIXS for approaching that limit.…”

Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

“…IXS experiments on hcp-Fe have been conducted over the last 15 years at the European Synchrotron Radiation Facility (ESRF) (Fiquet et al, 2001;Antonangeli et al, 2004b;Antonangeli et al, 2012) and more recently at the Advanced Photon Source (APS) (Mao et al, 2012) and at Spring-8 (Ohtani et al, 2013). This gives us the opportunity to compare measurements performed over time with the same instrument with similar experimental strategies, as well as results obtained by the same technique at different synchrotrons, often involving dissimilar experimental strategies.…”

“…These limitations become very critical when attempting to estimate the nature and amounts of light elements, whose presence as impurities in the inner core is required to modify the density and velocity of iron to match seismic observations. For instance, models based on linear extrapolations of V P -ρ relation argue for about 2 wt% silicon alloyed with iron in the inner core (Badro et al, 2007;Antonangeli et al, 2010), while a recent model using a power law for the V P -ρ extrapolation leads to 8 wt% silicon (Mao et al, 2012).…”