Limits of Water Storage in Stishovite at Deep Mantle Conditions

The deep Earth water cycle is a key process in Earth's evolution. Stishovite has been proposed as a major carrier of water from the surface into the lower mantle. However, its role remains unclear because its water solubility is controversial. We investigated the water solubility of Al-free stishovite at pressures of 33–38 GPa and temperatures of 700–1200 K using in situ X-ray diffraction in an advanced multianvil apparatus. Water solubility is high, around 3 wt%, at 700 K but decreases rapidly with increasing temperature and becomes negligible (<1,000 ppm), at temperatures above 1000 K. These results suggest that pure stishovite is unlikely to transport significant amounts of water into Earth's lower mantle under comparable pressure-temperature conditions. Previously reported high water contents in pure stishovite under deep Earth conditions likely reflect the solubility of post-stishovite or experimental artifacts.