Elasticity and Anisotropy of the Pyrite-Type FeO₂H-FeO₂ System in Earth’s Lowermost Mantle

Abstract

The pyrite-type FeO₂H-FeO₂ system has been experimentally confirmed to be stable in Earth’s lowermost mantle but there is limited information about its physical properties at high pressures constraining our understanding of its potential geophysical implications for the deep Earth. Here, static calculations demonstrate that the pyrite-type FeO₂H-FeO₂ system has a high density and Poisson’s ratio and ultra-low seismic velocities at conditions of Earth’s lowermost mantle. It provides a plausible mechanism for the origin of ultra-low velocity zones at Earth’s D″ layer. The incorporation of hydrogen in the pyrite-type FeO₂H-FeO₂ system tends to decrease the S wave velocity (V _S) but increase the bulk sound velocity (V _Φ), and can potentially explain the observed anti-correlation of V _S and V _Φ in the lowermost mantle. Additionally, FeO₂H exhibits nearly isotropic whereas FeO₂ is highly anisotropic, which may help understand some seismic anisotropies at the core-mantle boundary.

Keywords

FeO₂H / FeO₂ / ultra-low velocity zones / D″ layer / anisotropy

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