Superhydrous aluminous silica phases as major water hosts in high-temperature lower mantle

Significance

Dense aluminous silica minerals existing in subducted basaltic crusts are among the candidates for carrying water into the deep mantle, but their water solubility is still unclear due to the lack of direct evidence of water incorporation in their crystal structures. We quantitatively clarified that CaCl ₂-structured aluminous silica, called poststishovite, can contain weight percent levels of water in its crystal structure under topmost lower-mantle conditions by infrared spectroscopic observations. Our findings suggest that hydrous aluminous poststishovite is stable at lower-mantle conditions even in upwelling hot plumes. Owing to its wide pressure–temperature stability field, hydrous aluminous poststishovite plays an important role on the circulation of water between upper and lower mantles within cold subducting slabs and hot upwelling plumes.

Abstract

Water transported by subducted oceanic plates changes mineral and rock properties at high pressures and temperatures, affecting the dynamics and evolution of the Earth’s interior. Although geochemical observations imply that water should be stored in the lower mantle, the limited amounts of water incorporation in pyrolitic lower-mantle minerals suggest that water in the lower mantle may be stored in the basaltic fragments of subducted slabs. Here, we performed multianvil experiments to investigate the stability and water solubility of aluminous stishovite and CaCl ₂-structured silica, referred to as poststishovite, in the SiO ₂-Al ₂O ₃-H ₂O systems at 24 to 28 GPa and 1,000 to 2,000 °C, representing the pressure–temperature conditions of cold subducting slabs to hot upwelling plumes in the top lower mantle. The results indicate that both alumina and water contents in these silica minerals increase with increasing temperature under hydrous conditions due to the strong Al ³⁺-H ⁺ charge coupling substitution, resulting in the storage of water up to 1.1 wt %. The increase of water solubility in these hydrous aluminous silica phases at high temperatures is opposite of that of other nominally anhydrous minerals and of the stability of the hydrous minerals. This feature prevents the releasing of water from the subducting slabs and enhances the transport water into the deep lower mantle, allowing significant amounts of water storage in the high-temperature lower mantle and circulating water between the upper mantle and the lower mantle through subduction and plume upwelling. The shallower depths of midmantle seismic scatterers than expected from the pure SiO ₂ stishovite–poststishovite transition pressure support this scenario.