Hydrogen partitioning between stishovite and hydrous phase δ: implications for water cycle and distribution in the lower mantle

Abstract

Water is transported into the deep mantle by subducting slabs, playing important roles in mantle dynamics and evolution. An aluminous hydrous mineral, phase δ with a main component of AlOOH, has been considered an important water carrier in the lower mantle. Recent studies reported that SiO₂ stishovite can accommodate weight percent levels of water, indicating another important water carrier in the lower mantle. However, which mineral can mainly carry water is not clear yet. Recent hydrous phase relation studies reported that stishovite is depleted in alumina when coexisting with hydrous phase δ, in which water content of stishovite was not investigated. In this study, we investigated hydrogen partitioning between stishovite and hydrous phase δ at 24–28 GPa and 1000–1200 °C by means of Kawai-type multi-anvil press in combination with Fourier-transform infrared spectroscopy at ambient conditions on recovered samples. Fourier-transform infrared spectra of recovered stishovites showed that water contents of stishovite coexisting with hydrous phase δ were limited to up to ~ 500 ppm. This indicates that coexisting hydrous phase δ causes not only depletion in alumina but also in hydrogen in stishovite and therefore mainly transports water in a cold subducting slab. Once hydrous phase δ becomes thermally unstable, alumina and water contents in silica minerals are increased by the chemical reaction between SiO₂ and AlOOH, and aluminous silica minerals such as stishovite and CaCl₂-type phase will be a main water carrier in the lower mantle. Presence of small-scale seismic scatterers observed around 1900 km depth, which was considered to be caused by a transition from almost pure SiO₂ stishovite to CaCl₂-type phase, might also be able to be explained by the phase transition of stishovite coexisting with hydrous phase δ.