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Abstract 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 phasesδ‐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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Elastic Properties of the Pyrite‐Type FeOOH‐AlOOH System From First‐Principles Calculations
Abstract 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 (Ueff). The properties of pyrite‐type FeO2H are compared to that of pyrite‐type AlO2H, with which it likely forms a solid solution at high temperature, as well as the respective lower pressure CaCl2‐type polymorphs of both endmembers:ϵ‐FeOOH andδ‐AlOOH. Due to substantial differences in the CaCl2‐type → pyrite‐type structural transition pressures of these endmembers, the stabilities of the (Al,Fe)O2H solid solution polymorphs are anticipated to be compositionally driven at lower mantle pressures. As the geophysical properties of (Al,Fe)OOH are structurally dependant, interpretations regarding the contribution of pyrite‐type FeO2H to seismically observed features must take into account the importance of this broad phase loop. With this in mind, Fe‐rich pyrite‐type (Al,Fe)OOH may coexist with Al‐dominant CaCl2‐typeδ‐(Al,Fe)OOH in the deep Earth. Furthermore, pyrite‐type (Al0.5–0.6,Fe0.4–0.5)O2H can reproduce the reduced compressional and shear velocities characteristic of seismically observed ultra low velocity zones in the Earth's lowermost mantle while Al‐dominant but Fe‐bearing CaCl2‐typeδ‐(Al,Fe)OOH may contribute to large low shear velocity provinces.
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- 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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