The transport of hydrogen into Earth's deep interior may have an impact on lower mantle dynamics as well as on the seismic signature of subducted material. Due to the stability of the hydrous phases
The stability, structure, and elastic properties of pyrite‐type (FeS2structured) FeO2H were determined using density functional theory‐based computations with an internally consistent Coulombic self‐interaction term (
- NSF-PAR ID:
- 10450858
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 22
- Issue:
- 5
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract δ ‐AlOOH (delta phase), MgSiO2(OH)2(phase H), andε ‐FeOOH at high temperatures and pressures, their solid solutions may transport significant amounts of hydrogen as deep as the core‐mantle boundary. We have constrained the equation of state, including the effects of a spin crossover in the Fe3+atoms, of (Al, Fe)‐phase H: Al0.84Fe3+0.07Mg0.02Si0.06OOH, using powder X‐ray diffraction measurements to 125 GPa, supported by synchrotron Mössbauer spectroscopy measurements on (Al, Fe)‐phase H andδ ‐(Al, Fe)OOH. The changes in spin state of Fe3+in (Al, Fe)‐phase H results in a significant decrease in bulk sound velocity and occurs over a different pressure range (48–62 GPa) compared withδ ‐(Al, Fe)OOH (32–40 GPa). Changes in axial compressibilities indicate a decrease in the compressibility of hydrogen bonds in (Al, Fe)‐phase H near 30 GPa, which may be associated with hydrogen bond symmetrization. The formation of (Al, Fe)‐phase H in subducted oceanic crust may contribute to scattering of seismic waves in the mid‐lower mantle (∼1,100–1,550 km). Accumulation of 1–4 wt.% (Al, Fe)‐phase H could reproduce some of the seismic signatures of large, low seismic‐velocity provinces. Our results suggest that changes in the electronic structure of phases in the (δ ‐AlOOH)‐(MgSiO2(OH)2)‐(ε ‐FeOOH) solid solution are sensitive to composition and that the presence of these phases in subducted oceanic crust could be seismically detectable throughout the lower mantle. -
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Abstract Pyrite‐type FeO2H
x (P phase) has recently been suggested as a possible alternative to explain ultralow‐velocity zones due to its low seismic velocity and high density. Here we report the results on the congruent melting temperature and melt properties of P phase at high pressures from first‐principles molecular dynamics simulations. The results show that P phase would likely be melted near the core–mantle boundary. Liquid FeO2Hx has smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO2Hx could account for the observed seismic anomaly of ultralow‐velocity zones. However, to maintain the liquid FeO2Hx within the ultralow‐velocity zones against compaction requires special physical conditions, such as relatively high viscosity of the solid matrix and/or vigorous convection of the overlying mantle. -
null (Ed.)Abstract The high-pressure phases of oxyhydroxides (δ-AlOOH, ε-FeOOH, and their solid solution), candidate components of subducted slabs, have wide stability fields, thus potentially influencing volatile circulation and dynamics in the Earth’s lower mantle. Here, we report the elastic wave velocities of δ-(Al,Fe)OOH (Fe/(Al + Fe) = 0.13, δ-Fe13) to 79 GPa, determined by nuclear resonant inelastic X-ray scattering. At pressures below 20 GPa, a softening of the phonon spectra is observed. With increasing pressure up to the Fe 3+ spin crossover (~ 45 GPa), the Debye sound velocity ( v D ) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant v D . Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities ( v S , v P , and v Φ ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low- v Φ and low- v P anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between v S and v Φ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle.more » « less
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null (Ed.)Abstract Constraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth's lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa).more » « less
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Abstract Iron‐dominant metallic phases are likely the primary hosts for nitrogen in the reduced deep Earth, hence the storage of nitrogen in the lower mantle and the core is governed by the behavior of the Fe‐N‐C system at high temperatures and pressures. In this study, phase transitions and thermoelastic properties of iron carbonitrides were investigated at high pressure‐temperature conditions by diamond anvil cell experiments and first‐principles calculations. Experimental data revealed no phase transition in
ε ‐type Fe4(N0.6C0.4) or Fe7(N0.75C0.25)3up to 60 GPa at room temperature. At high temperature, Fe7(N0.75C0.25)3transforms into the Fe3C‐type phase at ∼27 GPa, and then into the Fe7C3‐type phase at ∼45 GPa, which is also corroborated by our theoretical calculations. We found that the phase stability of iron carbonitrides mainly depends on the N/C ratio, and the elastic properties of iron carbonitrides are dominantly affected by the Fe/(N+C) ratio. Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle. Some iron carbonitride inclusions in lower mantle diamonds could be the residue of the primordial mantle or originate from subducted nitrogen‐bearing materials, rather than iron‐enriched phases of the outer core. In addition, our experiments confirmed the existence of Fe7C3‐type Fe7C3‐Fe7N3solid solutions above 40 GPa. Fe7C3‐type Fe7(C, N)3has comparable density and thermoelastic properties to its isostructural endmembers and may be a promising candidate constituent of the Earth's inner core.
