Water solubility in nominally anhydrous minerals in a silica-rich system at the top lower mantle

Abstract

Although aluminous stishovite plays an important role in water transport and storage in the lower mantle, its water capacity in the top lower mantle is still poorly constrained. Here, we systematically investigated the water solubility in aluminous stishovite coexisting with bridgmanite, periclase, calcium ferrite-type phase, etc., in the presence of hydrous melts in a silica-rich MgO–Al₂O₃–SiO₂–H₂O system at pressures of 23–32 GPa and temperatures of 1600–2100 K using tungsten carbide anvils in a Walker-type large-volume press. Aluminous stishovite can accommodate significant amounts of water up to ∼3611 ppm wt., while the coexisting nominally anhydrous minerals have a very limited water storage capacity and are nearly dry. The water solubility in stishovite increases almost linearly from ∼296 to ∼3611 ppm wt. H₂O with increasing Al₂O₃ content from ∼0.3 to 3.0 wt. %. Our study further suggests that aluminous stishovite is a dominant phase for water transport and storage in a subducted oceanic crust at depths of 660–850 km, while the top peridotitic or pyrolitic lower mantle is nearly dry at least up to 850 km owing to the dry major phases of bridgmanite and periclase. The high water storage capacity of aluminous stishovite may prevent the occurrence of partial melting caused by the release of water from subducted slabs at the top of the lower mantle. The presence of hydrous aluminous stishovite may provide a plausible explanation for the high conductivity anomalies occasionally observed in some subducted slabs in the top lower mantle.