Elastic properties of hydrous ringwoodite at high-pressure conditions

The sound velocities and single-crystal elastic moduli of hydrous γ-Mg₂SiO₄ (ringwoodite) containing 2.3 wt% of H ₂O have been measured by Brillouin spectroscopy at high pressures to 23.4 GPa, spanning the pressure range in Earth's transition zone. The resulting pressure derivatives of the adiabatic bulk modulus, K′_S, and shear modulus, μ′, are 4.4(1) and 1.7(1) respectively. Compared with results for anhydrous ringwoodite, the pressure derivatives of the elastic moduli are consistent with an increase due to hydration of as much as 7% for the K′_S and 30% for μ′, depending on the data sets used for comparison. However, the gradients of velocity as a function of pressure for hydrous ringwoodite are significantly less than the corresponding gradients in the Earth's transition zone. We conclude that transition zone seismic velocity gradients are not due to "wet" ringwoodite, as previously speculated.