Shock-wave equation of state of rhyolite

Abstract

We have obtained new shock-wave equation of state (EOS) and release adiabat data for rhyolite. These data are combined with those of Swegle (1989, 1990) to give an experimental Hugoniot which is described by Us = 2.53(±0.08) + 3.393(±0.37)up for up < 0.48 km s-1, Us = 3.85(±0.05) + 0.65(±0.03)up for 0.48 ≤ up < 2.29 km s-1, Us = 1.52(±0.08) + 1.67(±0.02)up for 2.29 ≤ up < 4.37 km s-1, and Us = 3.40(±034) + 1.24(±0.06)up for up ≥ 4.37 km s-1, with ρ0 = 2.357 ± 0.052 Mg m-3. We suggest that the Hugoniot data give evidence of three distinct phases - both low- and high-pressure solid phases and, possibly, a dense molten phase. EOS parameters for these phases are ρ0 = 2.494 ± 0.002 Mg m-3, Ks0 = 37 ± 2 GPa, K′ = 6.27 ± 0.25, and γ = 1.0(V/V0) for the low-pressure solid phase; ρ0 = 3.834 ± 0.080 Mg m-3, Ks0 = 128 ± 20 GPa, K′ = 3.7 ± 1.4, and γ = 1.5 ± 0.5 for the solid high-pressure phase; and ρ0 = 3.71 ± 0.10 Mg m-3, Ks0 = 127 ± 25 GPa, K′ = 2.1 ± 1.0, and γ = 1.5 ± 1.0 for the dense liquid. Transition regions of the Hugoniot cover the ranges of 9-34 GPa for the low-pressure-high-pressure solid transition and 90-120 GPa for the high-pressure solid-liquid transition. Release paths from high-pressure states, calculated from the EOS parameters, suggest that the material remains in the high-pressure solid phase upon release. Release paths from both the high-pressure solid and liquid fall above the Hugoniot until the Hugoniot enters the low-pressure-high-pressure mixed phase region, when the release paths then cross the Hugoniot and fall below it, ending at significantly higher zero-pressure densities than that of the low-pressure phase. The low-pressure release paths fall very close to the Hugoniot. Estimates of residual heat deposition, based on shock-release path hysteresis, range from 20 to 60 per cent of the shock Hugoniot energy.