Shock compression and isentropic release of granite

S U M M A R Y New equation of state data for a weathered granite shocked to about 125 GPa are reported and combined with the Westerly granite data of McQueen, Marsh & Fritz (1967). The shock velocity (Us)-particle velocity (Up) relations can be fitted with two linear regressions: Us = 4.40 + 0.6Up for a range of Up up to about 2 km s-' and Us = 2.66 + 1.49Up for a range of about 2 to 5 km s-'. The third-order BirchMurnaghan equation of state parameters are KO, = 51-57 GPa and KAs = 1.4-1.8 for the low-pressure regime and KO, = 251 f 30 GPa and an assumed Kb, = 4 for the high-pressure regime. Compressive waveforms in dry and water-saturated granite were measured at 10-15GPa using the VISAR technique. The measured wave profiles were successfully modelled using a Maxwellian stress-relaxation material model. Water-saturated granite is characterized by a -25 per cent lower yield strength and a -75 per cent longer material relaxation time than dry granite. From measurements of partially released states in granite, it is proposed that the high-pressure forms of tectosilicates, including granite, relax isentropically to a metastable, intermediate phase characterized by a dense (about 3.7 g ~ m ~ ) , highly disordered, six-fold coordinated phase which is subsequently quenched to diaplectic glasses of density -2.3 g ~ m ~ , starting at pressure of -10 GPa. We develop an analytical model to describe the release isentropes in the mixed-phase regime which prescribe release to a glass phase with increasing transformation to the high-pressure phase. Hugoniot and post-shock energies and temperatures derived from the release isentropes are consistent with available data and theoretical expectations for quartz and granite.