Elastic properties of hydrous ringwoodite at high pressures

Phase transitions among olivine and its high-pressure polymorphs, wadsleyite and ringwoodite (α , β , and γ -Mg₂SiO₄), are thought to be responsible for the seismic velocity discontinuities in the transition zone of the mantle (400 ∼ 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 ∼3wt% H₂O 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% H₂O were synthesized in a multi-anvil press at 19 GPa and 1300° 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 ∼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 β and γ polymorphs of Mg₂SiO₄ at high pressure. As a result, if the β -> γ 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.