Shock-ramp of SiO2 melt.

SiO2 glass has been shock-melted at 100 GPa and then ramp compressed on the Z Accelerator at Sandia National Laboratories. In a shock-ramp experiment, a shockless ramp wave follows the shock impact, isentropically compressing the sample. This technique provides access to off-Hugoniot states, which are of importance to both materials and planetary sciences. Here we present results from shock-ramp experiments for two compositions of SiO2 glass, one with 1000 ppm by weight and one with <1ppm OH. We observe an increased particle velocity at the window-interface for SiO2 melt with 1000 ppm OH relative to the anhydrous sample, suggesting higher sound speed for the hydrous silicate melt. We describe how the lower shock impedance of molten silicate relative to the LiF window precludes the usual approach of iterative Lagrangian analysis to extract compressibility from shock-ramp velocity measurements. To understand the observation of higher velocity for the hydrated sample, and to guide development of a new shock-ramp data analysis approach, we present results from LAMMPS molecular dynamic simulations for SiO2 with 0-1wt.% H2O. These simulations show that the incorporation of water into SiO2 melt decreases the compressibility at constant pressure, consistent with the experimental observation of a stiffer response for the hydrated SiO2 melt. [ABSTRACT FROM AUTHOR]