Effect of water on shockless ramp compression of amorphous SiO₂ to upper mantle pressure

Earth's mantle has zones of anomalously low seismic velocities, some of which coincide with regions of known melt generation, such as subduction zones. Other low velocity zones (LVZs) have been found atop the mantle transition zone, but volatiles need to be present for mantle material to melt at such depths. Here we present shockless (ramp) compression data for two SiO₂ glasses (a-SiO₂) containing 0 ppm OH and 1000 ppm OH from 0 to ~9 GPa using the pulsed power machine Thor at Sandia National Laboratories. Ambient (1 atm) longitudinal sound velocities (V_p) of the dry and damp (1000 ppm OH) glasses, determined by GHz ultrasonic interferometry, are 5943 (±1.4) m/s and 5926 (±1.1) m/s, respectively. While the offset of V_p values persists throughout the entire pressure range, the behavior of both silicas is broadly similar; they both exhibit the anomalous decrease in V_p from 0 to ~2.5 GPa observed in experimental static compression studies of a-SiO₂, but the damp a-SiO₂ has slower V_p and V_p decreases up through ~3 GPa. Dry a-SiO₂ V_p speeds match static studies up to 2.5 GPa, but V_p values are lower than those in static experiments above 2.5 GPa. Accurate interpretation of observed seismic anomalies requires experimentally determined velocities of amorphous silicates. These experiments follow a quasi-isentrope, a similar thermodynamic profile to Earth's geotherm, indicating that our results may be used to interpret LVZs in the mantle.