Elasticity of β-Mg₂SiO₄ containing 1.2 wt% H₂O to 10 GPa and 600 K by ultrasonic interferometry with synchrotron X-radiation

We have measured the sound velocities and elasticity of synthetic polycrystalline β-Mg2SiO4 containing 1.2 wt% H2O to 10 GPa and 600 K using ultrasonic interferometry with synchrotron X-radiation. We determined sample length at high pressure and temperature using the sample's X-ray image and applied travel-time bond corrections appropriate to the experimental cell assembly configuration. Fitting the entire moduli data to third-order finite strain equations yields the adiabatic bulk [KS0 = 153.3(4) GPa] and shear [G0 = 101.8(2) GPa] moduli, their pressure derivatives (∂KS/∂P)T = 5.15(6) and (∂G/∂P)T = 1.68(3) and temperature derivatives (∂KS/∂T)P = -0.0179(9) GPa/K and (∂G/∂T)P = -0.0151(7) GPa/K. Comparing the bulk sound velocity contrast between the new hydrous wadsleyite data and olivine (0.38 wt% H2O) with seismic bulk sound velocity contrasts of 3.5% and 4.0% yields 53% and 60% olivine contents, respectively, assuming an isochemical mantle model of the Earth. The results suggest that a hydrous mantle transition zone with a pyrolite model composition could explain the 410 km seismic velocity jump.