SHOCK COMPRESSION AND ISENTROPIC RELEASE OF GRANITE

New equation of state data for a weathered granite shocked to about 12 5 GPa are reported and combined with the Westerly granite data of McQu een, Marsh and Fritz (1967). The shock velocity (U-s)-particle velocit y (U-p) relations can be fitted with two linear regressions: U-s = 4.4 0 + 0.6U(p) for a range of U-p up to about 2 km s(-1) and U-s = 2.66 1.49Upppp, for a range of about 2 to 5 km s(-1). The third-order Birc h-Murnaghan equation of state parameters are K-os = 51-57 GPa and K'(o s) = 1.4-1.8 for the low-pressure regime and K-os = 251 +/- 30 GPa and an assumed K'(os) = 4 for the high-pressure regime. Compressive wavef orms in dry and water-saturated granite were measured at 10-15 GPa usi ng the VISAR technique. The measured wave profiles were successfully m odelled using a Maxwellian stress-relaxation material model. Water-sat urated granite is characterized by a similar to 25 per cent lower yiel d strength and a similar to 75 per cent longer material relaxation tim e 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, intermediat e phase characterized by a dense (about 3.7 g cm(-3)), highly disorder ed, six-fold coordinated phase which is subsequently quenched to diapl ectic glasses of density similar to 2.3 g cm(-3), starting at pressure of similar to 10 GPa. We develop an analytical model to describe the release isentropes in the mixed-phase regime which prescribe release t o a glass phase with increasing transformation to the high-pressure ph ase. Hugoniot and post-shock energies and temperatures derived from th e release isentropes are consistent with available data and theoretica l expectations for quartz and granite.