The Influence of ɛ-to-Py FeOOH Phase Transition on the Lower Mantle Heterogeneities

Abstract

Hydrous minerals transported by cold subducting slabs to the lowermost mantle are believed to significantly influence mantle properties and the heterogeneities in the core-mantle boundary (CMB) region. FeOOH is one of the essential iron-bearing hydrous minerals whose high-pressure phases, ε-FeOOH and Pyrite-type FeOOH (Py-FeOOH), can remain stable at the pressure-temperature conditions pertaining to the deep lower mantle. Using the first-principles density functional theory (DFT) based methods, we compute the thermoelastic properties of ε-FeOOH and Py-FeOOH and investigate the role of these two minerals in the deep Earth. Our calculations suggest that the phase transition of ε-FeOOH to Py-FeOOH will result in an increase of V_P and V_S by roughly 9% and 10%, respectively, which can contribute to the positive velocity anomalies in the high-velocity zones (HVZs) found in the lower mantle regions below the eastern and western pacific. Our anisotropy studies for ε-FeOOH depict a decreasing trend of anisotropy with temperature, which might indicate the presence of ε-FeOOH in the upper regions of the LLSVPs. Due to its high density and thermoelastic properties, Py-FeOOH might contribute to ULVZs, ORPs and other features at the core-mantle boundary.