Phase stability and thermal equation of state of δ-AlOOH: Implication for water transportation to the Deep Lower Mantle

In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, delta-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that delta-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D '' layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of delta-AlOOH by the decomposition of phase Egg is associated with a similar to 2.1-2.5% increase in density (rho) and a similar to 19.7-20.4% increase in bulk sound velocity (V-Phi). The increase in rho across the phase Egg to delta-AlOOH phase transition can facilitate the subduction of delta-AlOOH to the lower mantle. Compared to major lower-mantle phases, delta-AlOOH has the lowest rho but greatest V-Phi, leading to an anomalous low rho/V-Phi ratio which can help to identify the potential presence of delta-AlOOH in the region. More importantly, water released from the breakdown of delta-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. delta-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone. (C) 2018 Elsevier B.V. All rights reserved.