Sound velocities of hydrous ringwoodite to 16GPa and 673K

To understand the effect of hydration on the sound velocities of major mantle minerals and to constrain the mantle's H ₂O budget, we have measured the single-crystal elastic moduli of hydrous ringwoodite, (Mg _1.633Fe ²⁺ _0.231Fe ³⁺ _0.026)Si _1.00H _0.179O ₄ with 1.1wt.% H ₂O using Brillouin scattering combined with X-ray diffraction in an externally-heated diamond anvil cell up to 16GPa and 673K. Up to 12GPa at 300K, the presence of 1.1wt.% H ₂O lowers the elastic moduli of ringwoodite by 5-9%, but does not affect the pressure derivatives of the elastic moduli compared to anhydrous ringwoodite. The reduction caused by hydration is significantly enhanced when temperatures are elevated at high pressures. At 12GPa, increasing temperature by δT=100K leads to a 1.3-2.4% reduction in the elastic moduli (C ₁₁, C ₁₂, and C ₁₄). Comparing our results with seismic observations, we have evaluated the potential H ₂O content in the lower part of the transition zone. Our results indicate that the observed seismic velocity anomalies and related depth depression of the 660-km discontinuity could be attributed to thermal variations together with the presence of ~0.1wt.% H ₂O.