High-Temperature Equation of State of FeH: Implications for Hydrogen in Earth's Inner Core

Tagawa, Shoh; Gomi, Hitoshi; Hirose, Kei; Ohishi, Yasuo

doi:10.1002/essoar.10508224.1

Cited by 7 publications

(34 citation statements)

References 30 publications

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“…(2022) showed that fcc FeH becomes less compressible above 41 GPa at room temperature, supporting the magnetic transition to non‐magnetic. We confirmed each time at 300 K before and after melting the sample that the unit‐cell volumes of the sample, including those of crystals that formed upon quenching temperature, were consistent with the compression curve of stoichiometric FeH (Tagawa et al., 2022).…”

Section: Resultsmentioning

confidence: 84%

“…The melting curve of FeH was obtained by fitting the thermodynamic model to these not‐melting and melting data along with those previously reported below 20 GPa (Sakamaki et al., 2009) (Figure 1). Its d T /d P slope becomes substantially larger due to the loss of magnetism above ∼40 GPa (Tagawa et al., 2022). The melting point of non‐magnetic FeH is higher than the eutectic melting temperature of FeH x (1 < x < 2) observed between 43 and 127 GPa (Hirose et al., 2019) (Figure 1).…”

Section: Resultsmentioning

confidence: 99%

“… Experimental data and calculated melting curve of stoichiometric FeH. Present experimental conditions of melting (red, inverse triangles) and non‐melting (blue, normal triangles) are plotted along with those where the unit‐cell volume was determined for solid FeH in a previous study (Tagawa et al., 2022) (small blue circles). Those reported by recent experiments (Piet et al., 2021) are also shown (small squares).…”

Section: Experimental Methodsmentioning

confidence: 99%

“…This method for determining pressure at high temperature based on the volume of the pressure medium has been validated in Tagawa et al. (2022) by simultaneously using NaCl and KCl; since the thermal expansivity of NaCl is much larger than that of KCl, the effect of temperature variation is larger, but NaCl gave pressures at high temperatures very similar to those by KCl.…”

Section: Experimental Methodsmentioning

confidence: 99%

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High‐Pressure Melting Curve of FeH: Implications for Eutectic Melting Between Fe and Non‐Magnetic FeH

et al. 2022

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Walsh et al., 2011), water may have been delivered to the growing Earth and Mars in large quantities. The high metal/silicate partition coefficient of H found in recent experimental and computational works suggests that most of the water transported to our planet was sequestered mainly as up to ∼1 wt% H in the core (Li et al., 2020;Tagawa et al., 2021;Yuan & Steinle-Neumann, 2020). Indeed, H-bearing iron (Fe) alloys can explain the observed density and seismic-wave velocities in both the liquid outer core (Umemoto & Hirose, 2015, 2020) and the solid inner core (He et al., 2022;Wang et al., 2021). In addition, the recent marsquake observations by the InSight probe revealed that the Martian core is less dense than previously thought, possibly including 1-2 wt% H in addition to sulfur (S) (Stähler et al., 2021).The Fe-FeH phase diagram is of great importance to understand the present state and composition of planetary metallic cores. However, it is still vaguely known. Fe-H alloys must be examined under high pressure, because

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Section: Resultsmentioning

confidence: 84%

Section: Resultsmentioning

confidence: 99%

Section: Experimental Methodsmentioning

confidence: 99%

Section: Experimental Methodsmentioning

confidence: 99%

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High‐Pressure Melting Curve of FeH: Implications for Eutectic Melting Between Fe and Non‐Magnetic FeH

et al. 2022

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“…Si concentration in the Fe-FeSi eutectic liquid has been shown to decrease from 11.5 wt% at 50 GPa to 8 wt% at 330 GPa 22 . On the other hand, the Fe-FeH eutectic composition (Fe + 0.8 wt% H) at ~50 GPa would remain similar at higher pressures because the temperature/pressure slope of the melting curve of stoichiometric FeH is comparable to that of Fe at >40 GPa 53 . The outer core composition should be within the liquidus field of Fe-the (Si, H)-depleted hcp phase-at 330 GPa (blue area in Fig.…”

Section: Discussionmentioning

confidence: 99%

Melting phase relations in Fe-Si-H at high pressure: Implications for Earth’s inner core crystallization and core light elements

Hikosaka

Tagawa

Hirose

et al. 2022

Preprint

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Hydrogen could be an important light element in planetary cores, but its effect on phase diagrams of iron alloys is not well known because the solubility of H in Fe is minimal at ambient pressure and high-pressure experiments on H-bearing systems have been challenging. Considering that silicon can be another major light element in planetary cores, here we performed melting experiments on the Fe-Si-H system at ~50 GPa and obtained the ternary liquidus phase relations and the solid/liquid partition coefficient, D of Si and H based on in-situ high-pressure X-ray diffraction measurements and ex-situ chemical and textural characterizations on recovered samples. Liquid crystallized hexagonal close-packed (hcp) (Fe0.93Si0.07)H0.25, which explains the observed density and velocities of the Earth’s solid inner core. The relatively high DSi = 0.94(4) and DH = 0.70(12) suggest that in addition to Si and H, the liquid outer core includes other light elements such as O, which is least partitioned into solid Fe and can thus explain the density difference between the outer and inner core. H and O, as well as Si, are likely to be major core light elements, supporting the sequestration of a large amount of water in the Earth’s core.

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Melting Experiments on Fe-O-H: Evidence for Eutectic Melting in Fe-FeH and Implications for Hydrogen in the Core

Oka¹,

Tagawa

Hirose

et al. 2022

Preprint

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We examined liquidus phase relations in Fe-O+/-H at ˜40 and ˜150 GPa, and subsolidus phase equilibria in Fe-FeH. While it has been speculated that Fe and FeH form continuous solid solution to core pressures, our experiments show the coexistence of the H-poor hcp and H-rich fcc phases in the Fe-FeH system. Considering higher melting temperature of stoichiometric FeH than that of Fe-FeH, it indicates eutectic melting between Fe and FeH. It is consistent with the liquidus phase diagram in Fe-O-H, which implies the Fe-FeH binary eutectic liquid composition of FeH0.42 at ˜40 GPa. We estimated the outer core liquid composition to be Fe + 2.9-5.2% O + 0.03-0.32% H + 0-3.4% Si + 1.7% S by weight, based on the liquidus phase relations, solid-liquid partitioning, and outer/inner core densities and velocities, indicating that O and either H or Si are important core light elements.

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High-Temperature Equation of State of FeH: Implications for Hydrogen in Earth's Inner Core

Cited by 7 publications

References 30 publications

High‐Pressure Melting Curve of FeH: Implications for Eutectic Melting Between Fe and Non‐Magnetic FeH

High‐Pressure Melting Curve of FeH: Implications for Eutectic Melting Between Fe and Non‐Magnetic FeH

Melting phase relations in Fe-Si-H at high pressure: Implications for Earth’s inner core crystallization and core light elements

Melting Experiments on Fe-O-H: Evidence for Eutectic Melting in Fe-FeH and Implications for Hydrogen in the Core

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