Hydrous aluminous silicas as major water hosts in the lower mantle
Authors: Katsura Tomoo , Ishii Takayuki , Criniti Giacomo , Ohtani Eiji , Purevjav Narangoo , Fei Hongzhan , Mao Ho-kwang 
Published: February 2023

The H2O incorporation into minerals changes the properties of minerals and rocks and affects the dynamics and evolution of the Earth&#8217;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&#176;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&#176;C to 10 wt.% at 2000&#176;C. Their H2O contents also increased with increasing temperature from 0.3 at 1700&#176;C to 1.0~1.1 wt.% at 1900&#176;C. The silica mineral was stishovite at temperatures lower than 1600~1700&#176;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+ &#8212; 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.