Ultrahigh-pressure elasticity of dense hydrous SiO2

Abstract Dense SiO2 phases such as stishovite and CaCl2-type SiO2 are the most elastically stiff minerals among major silicate and oxide minerals in the Earth’s mantle. Those phases have a significantly high water solubility compared to other nominally anhydrous minerals in the lower mantle. To date, there have been no direct measurements of sound velocity on dense hydrous SiO2. In this study, shear wave velocity of hydrous SiO2 was measured at room temperature up to 100 GPa using Brillouin light scattering spectroscopy in a diamond anvil cell. Compared to anhydrous SiO2, the shear wave velocity of hydrous SiO2 is significantly lower, but the difference of them gradually diminishes with increasing pressure. Water incorporation drastically softens SiO2 at lower pressure, whereas progressively make it stiffer with increasing pressure. These results suggest that water incorporation into SiO2 could produce the elastically distinct region with negative shear wave velocity anomalies in the lower mantle. This observation indicates that SiO2 can be a likely cause of water-induced stiffening of silicate minerals with increasing pressure.