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 (KS0) 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 KS0 = 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.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)