Sound velocities of (Al,Fe)-phase H and the seismic visibility of dense oxyhydroxides in Earth's lower mantle

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Strozewski, Benjamin T. ; Rautela, Ojashvi ; Buchen, Johannes ; Sturhahn, Wolfgang ; Ishii, Takayuki ; Ohira, Itaru ; Pardo, Olivia ; Zhao, Jiyong ; Toellner, Thomas S. ; Jackson, Jennifer M.:
Sound velocities of (Al,Fe)-phase H and the seismic visibility of dense oxyhydroxides in Earth's lower mantle.
In: Physics of the Earth and Planetary Interiors. Bd. 369 (2025) . - 107466.
ISSN 1872-7395
DOI: https://doi.org/10.1016/j.pepi.2025.107466

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Abstract

Oxyhydroxide phases in the (Al,Fe)OOH–MgSiO2(OH)2 system may form within oceanic lithosphere and transport hydrogen in their crystal structures into the lowermost mantle via cold, subducted slabs. In this work, we present new measurements of the seismic wavespeeds of the dense oxyhydroxide (Al,Fe)-phase H (Al0.84Fe0.073+Mg0.02Si0.06OOH) to 100 GPa constrained by nuclear resonant inelastic X-ray scattering, incorporating previous constraints on the equation of state of this phase. At 300 K and pressure greater than 70 GPa, (Al,Fe)-phase H exhibits high P-wave speeds (δlnVP >+12) and low S-wave speeds (δlnVS <−7) relative to the preliminary reference Earth model (PREM). Experimentally determined sound velocities are incorporated into a model of a hydrous metabasalt including (Al,Fe)-phase H and compared with the seismic wavespeeds of pyrolitic mantle along appropriate adiabats. Hydrous metabasalt may reproduce an anti-correlation of negative shear wave velocity and positive bulk sound velocity at the upper edges of large, low velocity provinces when compared to pyrolitic mantle but has similar wavespeeds to PREM in this region. Hydrous metabasalt with conceivable concentrations of (Al,Fe)-phase H can be distinguished from PREM in VS at mid-mantle depths (1100–1700 km) and in VP at shallower depths (750–1000 km). Subducted hydrous metabasalt could contribute to scattering of seismic waves across the depth interval of the post-stishovite transition, which may be affected by the formation of (Al,Fe)-phase H.