ELASTIC PROPERTIES OF HYDROUS RINGWOODITE (GAMMA-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 slight ly lower than that of ideal anhydrous ringwoodite. The H2O content det ermined 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 str ucture is cubic Fd3m, with a unit cell volume which is 0.51% larger th an that of the anhydrous ringwoodite. The adiabatic single-crystal ela stic moduli of hydrous ringwoodite, in GPa, are: C-11 = 281 +/- 6, C-4 4 = 117 +/- 4, C-12 = 92 +/- 5. The isotropic properties of hydrous ri ngwoodite 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 t he mantle transition zone relative to anhydrous ringwoodite. In additi on, our results show that the anisotropy is enhanced relative to anhyd rous ringwoodite. Using the present results, we discuss the 520 km sei smic discontinuity in a wet mantle transition zone. (C) 1998 Elsevier Science B.V. All rights reserved.