Limited stability of hydrous SiO2 stishovite in the deep mantle

The stability of minerals that can hold water is important for understanding the distribution and transportation of water in the Earth's deep interior. Water distribution in the lower mantle depends on the stability of water-bearing minerals in the subducting slab because minerals in the surrounding lower mantle have low water solubility. Recent studies have reported that pure SiO₂ high-pressure phases can hold large amounts of water (>3 wt%) however, their experimental results are contradictory regarding stability. In this study, the stability of hydrous SiO₂ stishovite in a water-saturated system was investigated at pressures of 10–30 GPa and temperatures reaching 1300 °C by in situ X-ray observation using a multi-anvil apparatus. The experiments revealed that the unit-cell volume of stishovite was significantly greater than that of anhydrous stishovite (by 3.8 % at the maximum) below 700 °C in the studied range of pressure, suggesting a high water content in stishovite (up to 5.4 wt% H₂O). However, the excess volume decreased rapidly at higher temperatures and the volume was approximately identical to anhydrous stishovite above 800 °C. Time-resolved measurements at constant temperatures of 450 and 500 °C, where water-induced excessive volume was observed, showed that the unit-cell volume shrank with time. This indicates that the dissolution of water in stishovite is a metastable phenomenon. These results indicate that SiO₂ stishovite in crustal materials subducting into the lower mantle is unlikely to retain >1 wt% of water as a stable phase.