Hydrous aluminous silicas as major water hosts in the lower mantle

The H₂O incorporation into minerals changes the properties of minerals and rocks and affects the dynamics and evolution of the Earth’s interior. The higher H₂O contents in plume-related magmas than in mid-oceanic ridge magmas suggest that deeper regions store more significant amounts of H₂O than shallower regions in the mantle. Paradoxically, however, the H₂O solubility in the lower-mantle minerals in ultramafic systems is limited. Therefore, we expect basaltic fragments of subducted slabs to store H₂O in the lower mantle. It has been suggested that silica minerals can be H₂O hosts in the basaltic systems under lower-mantle conditions, and alumina incorporation enhances their H₂O solubility. To determine the stability and water solubility of silica minerals under top-most lower-mantle conditions, the current study synthesised silica minerals in the SiO₂-Al₂O₃-H₂O systems at pressures of 24 and 28 GPa and temperatures of 1000 to 2000°C using a multi-anvil press. We identified phases present in the run products using a micro-focused X-ray diffractometer and measured their water solubility using an FT-IR spectrometer.

We found that the Al₂O₃ contents in the silica minerals increased with increasing temperature from 0.7~0.8 wt.% at 1000~1200°C to 10 wt.% at 2000°C. Their H₂O contents also increased with increasing temperature from 0.3 at 1700°C to 1.0~1.1 wt.% at 1900°C. The silica mineral was stishovite at temperatures lower than 1600~1700°C, whereas it was CaCl₂-structured silica, referred to as post-stishovite, at higher temperatures. Thus, post-stishovite contained much more significant amounts of H₂O than stishovite whose water content is consistent with previous reports.

The concomitant increases in H₂O and Al₂O₃ contents suggest that H₂O is incorporated via charge-coupled substitution of Si⁴⁺ — Al³⁺+H⁺ in these silica minerals. The current stability of post-stishovite in H₂O- and Al₂O₃-bearing systems is located at much lower pressures than in pure SiO₂ and H₂O-poor, Al₂O₃-bearing systems. In addition, the OH bands are more intense in the E//[010] direction than in the E//[100] direction. These observations imply that tilting of (Si, Al)O₆ octahedra around the c axis by the hydrogen bonding in the [010] direction may have stabilised poststishovite at lower pressures.

The increases in H₂O solubility in aluminous stishovite and poststishovite with temperature have a tremendous impact on the H₂O storage and transport in the mantle. The H₂O solubility in the other nominally anhydrous minerals decreases with increasing temperature. Dense hydrous magnesium silicates decompose with increasing temperature. Therefore, these minerals cannot be H₂O hosts or carriers in the deep mantle except for cold subduction zones. On the other hand, hydrous stishovite and poststishovite can store and transport H₂O in ambient mantle and even in plumes.

It has been considered that the stishovite-poststishovite transition causes seismic scattering in the mid-mantle. However, many seismic scatterers are located at 700 to 1900 km depths, which are too shallow for the stishovite-poststishovite transition in the pure SiO₂ system. However, we found that the Al₂O₃ and H₂O incorporations lower the transition pressure to 24 GPa, i.e., 700 km depth. Hence, observing the seismic scatterers in the mid-mantle supports significant H₂O storages in aluminous poststishovite.