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-Fo₁₀₀) and hydrous olivine (hy-Fo₉₇) containing 0.8-0.9 wt% H₂O. The samples, synthesized at 12 GPa and 1250°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 (G₀) moduli of hy-Fo₁₀₀ are 125.7(±0.2) GPa and 79.8(±0.1) GPa, respectively. For hy-Fo₉₇, we obtain K_S0 = 124.4(±0.4) GPa and G₀ = 75.3(±0.3) GPa. Compared with anhydrous forsterite, the combined effects of 3 mol% Fe and 0.8 wt% H₂O reduce bulk and shear moduli by 3.5(±0.3)% and 7.5(±0.4)% respectively, with greater reductions expected for more iron-rich Fo₉₀ 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.