Pressure‐induced magnetic transition and sound velocities of Fe<sub>3</sub>C: Implications for carbon in the Earth's inner core

Gao, Lili; Chen, Bin; Wang, Jingyun; Esen, E.; Zhao, Jiyong; Lerche, M.; Sturhahn, W.; Scott, H. P.; Huang, Fang; Ding, Yang; Sinogeikin, Stanislav V.; Lundstrom, Craig C.; Bass, Jay D.; Li, Jie

doi:10.1029/2008gl034817

Cited by 74 publications

(96 citation statements)

References 27 publications

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“…In the past few decades, various techniques, including synchrotron XRD (29,30), NRIXS (11,22,36,37), HERIX (10, 31-33, 38, 39), and impulsive stimulated light scattering (ISLS) (40), combined with DACs, have been applied to measure the V P of Fe-light element alloys at high pressures (Fig. 3A).…”

Section: Resultsmentioning

confidence: 99%

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

Mao

Lin

Liu

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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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...

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Section: Resultsmentioning

confidence: 99%

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

Mao

Lin

Liu

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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101

138

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“…Cementite iron carbide (Fe 3 C) was initially proposed as the most likely carbon vehicle [6]. However, consideration of the pressure-induced transition of Fe 3 C from a ferromagnetic to a nonferromagnetic state, as evidenced by ab initio calculations [7], sound velocity measurements [8], and high pressure experiments [9], results in conflicting evidence.…”

Section: Introductionmentioning

confidence: 95%

First-principles calculations of properties of orthorhombic iron carbideFe7C3at the Earth's core conditions

et al. 2015

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A recently discovered phase of orthorhombic iron carbide o-Fe 7 C 3 [Prescher et al., Nat. Geosci. 8, 220 (2015)] is assessed as a potentially important phase for interpretation of the properties of the Earth's core. In this paper, we carry out first-principles calculations on o-Fe 7 C 3 , finding properties to be in broad agreement with recent experiments, including a high Poisson's ratio (0.38). Our enthalpy calculations suggest that o-Fe 7 C 3 is more stable than Eckstrom-Adcock hexagonal iron carbide (h-Fe 7 C 3 ) below approximately 100 GPa. However, at 150 GPa, the two phases are essentially degenerate in terms of Gibbs free energy, and further increasing the pressure towards Earth's core conditions stabilizes h-Fe 7 C 3 with respect to the orthorhombic phase. Increasing the temperature tends to stabilize the hexagonal phase at 360 GPa, but this trend may change beyond the limit of the quasiharmonic approximation.

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“…On the contrary, V P derived from the phonon density of states measured by NRIXS up to 153 GPa (Mao et al, 2001) plots higher than shock wave determination (Brown and McQueen, 1986), indicating possible temperature effects, and extrapolates at core pressures to V P higher than PREM. Later NRIXS and IXS studies, with progressively improved beamline performance (flux, focusing optics), improved statistics and improved sample environment (hydrostaticity and/or texture characterization), focused on anisotropy (Antonangeli et al, 2004b;Lin et al, 2010), high-temperature effects (Lin et al, 2005;Kantor et al, 2007;Antonangeli et al, 2008;Antonangeli et al, 2012;Ohtani et al, 2013), or effect of nickel and/or light element inclusion (Lin et al, 2003;Badro et al, 2007;Kantor et al, 2007;Gao et al, 2008;Fiquet et al, 2009;Antonangeli et al, 2010;Shibazaki et al, 2012;Kamada et al, 2014). In view of the specific characteristics of the two techniques and of the respective data analysis, today's general consensus is that, for measurements on polycrystalline samples, IXS provides more reliable V P and NRIXS more reliable V S (the Debye velocity measured directly by NRIXS is more heavily weighted in the determination of V S , while V P determination is more dependent on the used equation of state; in comparison, IXS provides a direct determination of V P while V S depends on the choice of equation of state).…”

Section: Experimental Techniques For Sound Velocity Determination Undmentioning

confidence: 99%

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

Antonangeli

Ohtani

2015

Prog. in Earth and Planet. Sci.

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Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth's core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth's inner core composition.

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Pressure‐induced magnetic transition and sound velocities of Fe₃C: Implications for carbon in the Earth's inner core

Cited by 74 publications

References 27 publications

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

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

First-principles calculations of properties of orthorhombic iron carbideFe7C3at the Earth's core conditions

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

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Pressure‐induced magnetic transition and sound velocities of Fe3C: Implications for carbon in the Earth's inner core

Cited by 74 publications

References 27 publications

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

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

First-principles calculations of properties of orthorhombic iron carbideFe7C3at the Earth's core conditions

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

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Pressure‐induced magnetic transition and sound velocities of Fe₃C: Implications for carbon in the Earth's inner core