SINGLE-CRYSTAL ELASTIC PROPERTIES OF PHASE A TO 12 GPA

The sound velocities and single-crystal elastic constants of dense hydrous magnesium silicate phase A, Mg7Si2O8(OH)6, were measured by Brillouin scattering up to 12 GPa in a diamond anvil cell. The single-crystals of phase A used in this study were synthesized at 12 GPa and 1150 ¨¬C in a 5000-tons multi-anvil press at Bayerisches Geoinstitut (Germany). The pressure-dependence of single-crystal elastic moduli (Cij), as well as the aggregate adiabatic bulk (Ks) and shear (m) moduli were obtained. At ambient conditions, the adiabatic bulk and shear moduli of phase A are 106(2) GPa and 62(1) GPa, respectively (VRH averages). The present acoustic results resolve discrepancies between the bulk moduli obtained in earlier compression studies. The compressibility of phase A is anisotropic, with the a-axis being 20% more compressible than the c-axis, in agreement with previous static compression data. Our present results help to elucidate the effect of water incorporation on the elastic properties (Ks and m) of mineral phases along the forsterite-brucite join. The effects of pressure on the elasticity of phase A and the implications for the storage of water in the Earth's mantle and transition zone will be discussed.