HR: 0830h
AN: V31D-0971    [PDF]
TI: Elastic properties of hydrous ringwoodite at high pressures
AU: * Wang, J
EM: jwang11@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 245 NHB 1301 W. Green St., Urbana, IL 61801 United States
AU: Sinogeikin, S V
EM: sinegeik@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 245 NHB 1301 W. Green St., Urbana, IL 61801 United States
AU: Inoue, T
EM: inoue@sci.ehime-u.ac.jp
AF: Ehime University, Bunkyo-cho 2-5, Matsuyama, 790-8577 Japan
AU: Bass, J D
EM: jaybass@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 245 NHB 1301 W. Green St., Urbana, IL 61801 United States
AB: Phase transitions among olivine and its high-pressure polymorphs, wadsleyite and ringwoodite ($\alpha$, $\beta$, and $\gamma$-Mg$_{2}$SiO$_{4}$), are thought to be responsible for the seismic velocity discontinuities in the transition zone of the mantle (400 $\sim$ 660 km). Measurements of sound velocities in these minerals are therefore necessary to interpret seismological information on these discontinuities in terms of transition zone composition and temperature. Recent studies have demonstrated that ringwoodite can contain up to $\sim$3wt% H$_{2}$O by weight, thereby making it possible to store vast quantities of water in the transition zone. It has also been shown that the incorporation of water into the crystal structure of ringwoodite results in a substantial decrease in the elastic moduli. However, inferences concerning the water content of the transition zone are still highly uncertain because there is little experimental data on the elasticity of hydrous ringwoodite under high-pressure and high temperature conditions.\\ We have therefore undertaken a study of the sound velocities and single-crystal elastic moduli of hydrous ringwoodite at high pressure by Brillouin spectroscopy. Single crystals of hydrous Mg end-member ringwoodite containing 2.3wt% H$_{2}$O were synthesized in a multi-anvil press at 19 GPa and $1300\deg$C. Samples were loaded into a Merrill-Bassett-style diamond anvil cell with various pressure media. We have thus far obtained results for the single-crystal elastic moduli, the bulk modulus K$_{S}$, and the shear modulus G to P $\sim$14 GPa. Our results indicate that the pressure derivatives of the adiabatic bulk modulus and shear modulus are very similar to the values for dry ringwoodite. We conclude that water does not significantly affect the pressure derivatives K$_{S}$' or G'. Therefore, hydration does not significantly change the velocity contrast between the $\beta$ and $\gamma$ polymorphs of Mg$_{2}$SiO$_{4}$ at high pressure. As a result, if the $\beta$ $\rightarrow$ $\gamma$ transition is responsible for a seismic discontinuity near 520 km depth, the magnitude of this discontinuity will not depend on the presence of water in the transition zone. Velocity gradients in at least the lower part of the transition zone are also insensitive to the degree of hydration.
DE: 1025 Composition of the mantle
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting