Shock induced melting of silica: Hugoniot states, sound speeds, and Grüneisen parameter to 154 GPa*

In the Earth, free silica is expected to play a role in cold subducting slabs of eclogitic/basaltic composition. For this reason, silica phases (fused silica, -quartz, stishovite, etc.) have been extensively studied in both static and dynamic compression experiments. Although well studied in the solid state and at extreme conditions, there is a gap in understanding the response of liquid silica at lower mantle pressures. Therefore, we conducted plate impact experiments using laser interferometry (VISAR) to measure wave profiles in an overtaking rarefaction experimental configuration. The principal Hugoniot states and longitudinal sound speeds in shock compressed fused silica were determined across the solid-liquid boundary, reaching pressures up to 154 GPa. We find that shock compressed fused silica transitions from the crystalline stishovite phase to the fully liquid phase at 80 GPa, a ~10% higher transition pressure than suggested from previous temperature measurements. Our sound speed and Hugoniot measurements provide the most precise determination of the Grüneisen parameter (g) in a liquid silicate to date and show that for silica, a proxy for magma ocean-type silicate liquids, g increases with compression from 80 GPa to 154 GPa.