The Elastic Properties of β-Mg2SiO4 Containing 0.73 wt.% of H2O to 10 GPa and 600 K by Ultrasonic Interferometry with Synchrotron X-Radiation

We measured the elastic velocities of a synthetic polycrystalline beta -Mg2SiO4 containing 0.73 wt.% H2O to 10 GPa and 600 K using ultrasonic interferometry combined with synchrotron X-radiation. Third-order Eulerian finite strain analysis of the high P and T data set yielded K-so = 161.5(2) GPa, G(o) = 101.6(1) GPa, and (partial derivative K-s/partial derivative P)(T) = 4.84(4), (partial derivative G/partial derivative P)(T) = 1.68(2) indistinguishable from K-so = 161.1(3) GPa, G(o) = 101.4(1) GPa, and (partial derivative K-s/partial derivative P)(T) = 4.93(4), (partial derivative G/partial derivative P)(T) = 1.73(2) from the linear fit. The hydration of the wadsleyite by 0.73 wt.% decreases K-s and G moduli by 5.3% and 8.6%, respectively, but no measurable effect was noted for (partial derivative K-s/partial derivative P)(T) and (partial derivative G/partial derivative P)(T). The temperature derivatives of the K-s and G moduli from the finite strain analysis (partial derivative K-S/partial derivative T)(P) = -0.013(2) GPaK(-1), (partial derivative G/partial derivative T)(P) = -0.015(0.4) GPaK(-1), and the linear fit (partial derivative K-S/partial derivative T)(P) = -0.015(1) GPaK(-1), (partial derivative G/partial derivative T)(P) = -0.016(1) GPaK(-1) are in agreement, and both data sets indicating the |(partial derivative G/partial derivative T)(P)| to be greater than |(partial derivative K-S/partial derivative T)(P)|. Calculations yield Vp(alpha-beta) = 9.88% and VS(alpha-beta) = 8.70% for the hydrous beta -Mg2SiO4 and hydrous alpha -Mg2SiO4, implying 46-52% olivine volume content in the Earth's mantle to satisfy the seismic velocity contrast V-s = V-P = 4.6% at the 410 km depth.