Sound velocities and elastic constants of iron-bearing hydrous ringwoodite

The sound velocities and single-crystal elastic constants of Fo(89) hydrous ringwoodite (gamma-Mg1.7Fe0.22H0.16SiO4) containing similar to10,000 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-Gamet (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 Fo(89) ringwoodite at ambient conditions are (in GPa): c(11) = 298 +/- 13, c(44) = 11 +/- 6, and c(12) 115 +/- 6. Hydration of Fosimilar to(90) ringwoodite to 1 wt.% H2O reduces the adiabatic bulk (K-0S 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 degreesC, 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 Fosimilar to(90) ringwoodite. P- and S-wave velocities of the lower transition zone in PREM are consistent with a hydrated ringwoodite-rich composition. (C) 2004 Elsevier B.V. All rights reserved.