Ramp compression of amorphous SiO₂ at upper mantle pressures

Observations of lower-than-average seismic wave velocities in the upper mantle at plate boundaries, the lithosphere-asthenosphere boundary, and at the upper and lower boundaries of the mantle transition zone, at 410 and 660 km depths respectively, have been attributed to the presence of silicate melts.^1,2 However, interpretation of seismic signatures including the composition of melts in the Earth’s upper mantle requires experimentally determined sound velocity at high pressure. Here we present results for SiO₂, the primary component of natural silicate magma and the fully polymerized endmember, as an analog for silicate melts generated in the mantle. We perform shockless (ramp) compression on the pulsed-power machine Thor at Sandia National Laboratories to obtain the Lagrangian sound velocity and density of SiO₂ glass continuously from 0 - 9 GPa. These experiments probe a quasi-isentropic pressure-temperature path, similar to the mantle adiabat. Below 7 GPa, our data show similar pressure-dependence on sound velocity to static compression data. Above 7 GPa, our data exhibit slower sound velocities than static data. To interpret the experimental data, we will calculate the principal isentrope from Sesame 7361 tabular equation of state.



[1] Schmerr, N. (2012). Science.

[2] Song, T.-R. A., Helmberger, D. V., & Grand, S. P. (2004). Nature.



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