Sound velocities in stishovite under shock compression up to 72 GPa

Stishovite (SiO₂) is expected to comprise up to 20 vol.% of basaltic slabs that plunge beyond 660 km (23 GPa) depth into the lower mantle [1]. To detect its presence through the interpretation of seismic data, constraints on seismic-wave velocities at the relevant pressure and temperature conditions are needed. Understanding the properties of SiO₂ at high pressure and temperature is also motivated by the increase in the coordination of Si from 4 to 6 between the upper and lower mantle. This coordination increase accounts for the high density of stishovite and is likely responsible for the change in properties of lower mantle silicates in general. There is thus a strong motivation to quantify the properties of SiO₂ directly under lower mantle conditions to both understand the behavior of free silica and to develop our understanding of this fundamental component of silicates.

Recent in situ X-ray diffraction experiments show that fused silica crystallizes to stishovite under shock compression to 36 GPa and above [2]. Shock-wave studies are particularly valuable for the study of mantle minerals because the pressure and temperature conditions obtained under shock compression are close to an expected mantle geotherm. Moreover, the velocity of the release wave that follows the shock provides a measurement of the speed of sound at the pressure and temperature conditions behind the shock. Here, we present sound-speed measurements in stishovite formed during shock-compression of fused silica. The plate-impact experiments were done at Washington State University using a two-stage light-gas gun combined with time-resolved laser interferometry. Preliminary results reveal that the longitudinal sound speed in stishovite formed by shock-compression of fused silica is 11.5 km/s at 44 GPa, a value slower than predicted by ab initio models [3]. The sound speed is observed to decrease to 10 km/s at 72 GPa. Our experiments aim to constrain sound velocities in stishovite from 34 - 72 GPa, understand the decrease in sound speed at 72 GPa, and examine the shock-response of fused silica up to 100 GPa.

References:

[1] Wood et al. Elements 9, 421-42 (2013).

[2] Tracy et al. Phys. Rev. Lett. 120, 135702 (2018).

[3] Yang and Wu, Earth Planet. Sci. Lett. 404, 14-21 (2014).