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Effect of Water on the Sound Velocities of Ringwoodite in the Transition Zone

High-pressure elasticity studies will play a central role in efforts to constrain the potential hydration state of the Earth's mantle from seismic observations. Here we report the effects of 1 wt% H 2 O (as structurally bound OH) on the sound velocities and elastic moduli of single-crystal ringwoodite of Fo 90 composition, thought to be the dominant phase in the deeper part of the transition zone between 520 and 660-km depth. The experiments were made possible through development of a GHz-ultrasonic interferometer used to monitor P and S-wave travel times through micro-samples (30-50 μm thickness) under hydrostatic compression in the diamond-anvil cell. The velocity data to ∼10 GPa indicate that hydrous ringwoodite supports 1-2% lower shear-wave velocities than anhydrous ringwoodite at transition zone pressures, though elevated pressure derivatives (K' = 5.3 ± 0.4 and G' = 2.0 ± 0.2) bring calculated hydrous P-velocities close to anhydrous values within their mutual uncertainties above ∼12 GPa. Corresponding V P /V S ratios are elevated by ∼2.3% and not strongly dependent on pressure. Velocities for hydrous ringwoodite are calculated along a 1673 K adiabat using finite-strain theory and compared with existing data on anhydrous ringwoodite and various radial seismic models. It may be possible to distinguish hydration from temperature anomalies by low S-velocities associated with "normal" P-velocities and accompanying high V P /V S ratios. The presence of a broadened and elevated 410-km discontinuity, together with depressed 660-km discontinuities and intervening low S-wave anomalies along with high V P / V S ratios are the most seismologically diagnostic features of hydration considering the available information from mineral physics.