Shock wave compression of single-crystal forsterite

Abstract

Hugoniot equation of state measurements have been performed on pure synthetic single-crystal forsterite (Mg_2SiO_4) in the pressure range 70–160 GPa (0.7–1.6 Mbar). These and earlier data for polycrystalline forsterite are compared with theoretical Hugoniots for the assemblages 2MgO (rocksalt) + SiO_2 (stishovite) and MgO (rocksalt) + MgSiO_3 (perovskite). The densities attained by single-crystal forsterite at pressures in excess of 120 GPa are greater than those expected in the event of shock-induced transformation to the isochemical oxide mixture. A similar test of the hypothesis of shock-induced transformation to the perovskite-bearing assemblage is sensitive to the choice of MgSiO_3 (perovskite) bulk modulus. Recent static compression measurements of Yagi et al. (1978) yield a K_(0T) of 286 GPa (for K_(0T)′ = 5), which, along with other elastic and thermodynamic parameters, suggests that shocked forsterite may be more dense than the perovskite-bearing assemblage. Crystalline phases of up to 5% greater zeropressure density or equally dense short-range-order-only phases may well be involved. Alternatively, the use of an isentropic bulk modulus of 250 GPa (estimated by Liebermann et al., 1977) for MgSiO_3 (perovskite) allows consistency between the data and the calculated MgO + MgSiO_3 (perovskite) Hugoniot for a reasonable choice (∼3.8) of K_(0s)′ for the latter phase. The new forsterite data along with high-pressure Hugoniot data for other olivines and olivinitic rocks define a smooth isobaric variation of Hugoniot density with composition. It is shown that an estimated pyrolite (Ringwood, 1975) Hugoniot density of 5.31 g/cm^3 at 120 GPa is ∼2% less dense than inferred from typical lower mantle density profiles.