Sound velocities of hydrous ringwoodite to 16 GPa and 673 K

To understand the effect of hydration on the sound velocities of major mantle minerals and to constrain the mantle's H2O budget, we have measured the single-crystal elastic moduli of hydrous ringwoodite, (Mg1.633Fe2 +0.231Fe3 +0.026)Si1.00H0.179O4 with 1.1 wt.% H2O using Brillouin scattering combined with X-ray diffraction in an externally-heated diamond anvil cell up to 16 GPa and 673 K. Up to 12 GPa at 300 K, the presence of 1.1 wt.% H2O 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 12 GPa, increasing temperature by ΔT = 100 K leads to a 1.3–2.4% reduction in the elastic moduli (C11, C12, and C14). Comparing our results with seismic observations, we have evaluated the potential H2O 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 ~ 0.1 wt.% H2O.

► We have studied the elasticity of Fe-bearing ringwoodite with 1 wt.% H2O at high P–T. ► Water lowers the elasticity but does not affect the pressure derivatives at 300 K. ► The reduction caused by hydration is enhanced when T is elevated at high P. ► Variations in T with ~ 0.1 wt.% H2O may explain the seismic anomaly at 660-km depth.