Effects of hydration on the elastic properties of olivine

Water, dissolved as hydroxyl (OH)(-) into the solid silicate minerals of the upper mantle can reduce adiabatic wave speeds through associated defects. Here we report Brillouin spectroscopy measurements of the sound velocities and single-crystal elastic constants of hydrous forsterite (hy-Fo(100)) and hydrous olivine (hy-Fo(97)) containing 0.8-0.9 wt% H(2)O. The samples, synthesized at 12 GPa and 1250 degrees C, represent nearly the maximum storage capacity of water in olivine at conditions of 350 400 km depth. The adiabatic bulk (K(S0)) and shear (G(0)) moduli of hy-Fo(100) are 125.7(+/- 0.2) GPa and 79.8(+/- 0.1) GPa, respectively. For hy-Fo(97), we obtain K(S0) = 124.4(+/- 0.4) GPa and G(0) = 75.3(+/- 0.3) GPa. Compared with anhydrous forsterite, the combined effects of 3 mol% Fe and 0.8 wt% H(2)O reduce bulk and shear moduli by 3.5(+/- 0.3)% and 7.5(+/- 0.4)% respectively, with greater reductions expected for more iron-rich Fo(90) mantle compositions. Although lattice preferred orientation (LPO) studies have not been carried out under relevant conditions of water or pressure, analysis of idealized single-crystal anisotropy for various known LPO types predicts up to 3% higher S-wave splitting anisotropy in hydrous olivine crystals aligned according to A- type and E-type fabrics, but no change in S-wave splitting anisotropy for B-type and C-type fabrics.