Elastic properties of hydrous ringwoodite (γ-phase) in Mg2SiO4

Single-crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 GPa and 1300 degrees C. The Mg/Si ratio is determined to be similar to 1.95 by EPMA, which is slightly lower than that of ideal anhydrous ringwoodite. The H2O content determined by SIMS is similar to 2.2 wt%, resulting in the stoichiometry Mg1.89Si0.97O4H0.33.The lattice parameter (a = 8.0786 +/- 4 Angstrom) and elastic constants were determined from a single crystal with X-ray diffractometry and Brillouin scattering spectroscopy. The crystal structure is cubic Fd3m, with a unit cell volume which is 0.51% larger than that of the anhydrous ringwoodite. The adiabatic single-crystal elastic moduli of hydrous ringwoodite, in GPa, are: C-11 = 281 +/- 6, C-44 = 117 +/- 4, C-12 = 92 +/- 5. The isotropic properties of hydrous ringwoodite are K-VRH = 155 +/- 4 and G(VRH) = 107 +/- 3 which are about 16 and 10% smaller than those of anhydrous ringwoodite. Thus hydrous ringwoodite, if present, drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite. In addition, our results show that the anisotropy is enhanced relative to anhydrous ringwoodite. Using the present results, we discuss the 520 km seismic discontinuity in a wet mantle transition zone. (C) 1998 Elsevier Science B.V. All rights reserved.