Sound Velocities of Tremolite at High Pressure: Implications for Detecting Water on Venus

While water may be stored in Venus' interior in the form of hydrous minerals, the velocity structure on Venus has been inferred based on anhydrous conditions. Due to its high decomposition temperature, tremolite could potentially store water on Venus, and thus, it is important to assess if the water-bearing tremolite could potentially have a distinct signal in velocity profiles of the Venusian lithosphere that future missions could measure. In this study, we measured the elastic properties of tremolite polycrystalline samples at high pressure up to 6 GPa using an ultrasonic interference method. Our results show that both VP and VS for tremolite increase linearly with pressure, and the values are much lower than those of nominally anhydrous minerals. The VP/VS ratio of 1.59-1.73 is less than that of most hydrous minerals. Combining the thickness of the crust and mineralogic models in previous studies, we attempt to build a possible wave velocity structure of Venus bearing tremolite deep to 100 km. Our results indicate that under cold temperature gradients, tremolite decomposes at 56 km resulting in a jump of 3.0% and 2.2% in VP and VS respectively, while under hot temperature gradients, tremolite decomposes at 35.2 km resulting in a jump of 3.1% and 2.2% in VP and VS respectively. When seismic velocity profiles for Venus become available through future missions, these theoretical profiles could provide insight into the potential presence of tremolite in the lithosphere of Venus and therefore the global water budget of the planet.