Sound velocities and elastic constants of iron-bearing hydrous ringwoodite

The sound velocities and single-crystal elastic constants of Fo89 hydrous ringwoodite (γ-Mg1.7Fe0.22H0.16SiO4) containing ∼10,000 ppm by weight (1 wt.%) H2O have been determined from seven separate pure-mode travel-time measurements using gigahertz ultrasonic interferometry. The experiments feature a new Yttrium–Aluminum–Garnet (YAG) P-to-S conversion shear buffer rod (BR), capable of producing pure ultrasonic shear waves with known polarization in the region of 0.5–2.0 GHz. To our knowledge they are the first such single-crystal ultrasonic measurements on a high-pressure phase recovered from the multi-anvil press. The cubic single-crystal elastic constants of hydrous Fo89 ringwoodite at ambient conditions are (in GPa): c11=298±13, c44=112±6, and c12=115±6. Hydration of Fo∼90 ringwoodite to 1 wt.% H2O reduces the adiabatic bulk (K0S=176±7 GPa) and shear (G=103±5 GPa) moduli by about 6 and 13%, roughly equivalent to raising the temperature at room pressure by 600 and 1000 °C, respectively. Assuming a linear trend with hydration, we calculate that P- and S-wave velocities are reduced by about 40 m/s for every 1000 ppm weight (0.1 wt.%) H2O added to Fo∼90 ringwoodite. P- and S-wave velocities of the lower transition zone in PREM are consistent with a hydrated ringwoodite-rich composition.