Single-crystal elasticity of wadsleyites, β -Mg₂SiO₄, containing 0.37-1.66 wt.% H₂O (DOI:10.1016/j.epsl.2007.10.045)

The presence of hydrogen can affect elastic properties and seismic velocities of minerals in the Earth's upper mantle. In this study, the second-order elastic constants of hydrous wadsleyites containing 0.37, 0.84, and 1.66 wt.% H₂O were determined by Brillouin scattering at ambient conditions. Measurements were performed on at least three independent crystal planes for each composition. The aggregate bulk modulus, K_S0, and shear modulus, G₀, were calculated using VRH (Voigt-Reuss-Hill) averages. The results are: K_S0 = 165.4(9) GPa, G₀ = 108.6(6) GPa for wadsleyite with 0.37 wt.% H₂O; K_S0 = 160.3(7) GPa, G₀ = 105.3(6) GPa for 0.84 wt.% H₂O; K_S0 = 149.2(6) GPa, G₀ = 98.6(4) GPa for 1.66 wt.% H₂O. We find that the bulk and shear moduli of hydrous wadsleyites decrease linearly with water content according to the following relations (in GPa): K_S0 = 170.9(9) - 13.0(8) C_H2O, G₀ = 111.7(6) - 7.8(4) C_H2O, where C_H2O is the H₂O weight percentage. Compared with anhydrous wadsleyite, addition of 1 wt.% H₂O will lead to a 7.6% decrease in the bulk modulus, and a 7.0% decrease in the shear modulus. Using these results, we examine the velocity contrast between hydrous olivine and wadsleyite at ambient conditions for an Fe-free system assuming an H₂O partition coefficient between wadsleyite and olivine of 3. The velocity contrast in compressional and shear velocity between wadsleyite and olivine ranges from 12-13% for an H₂O-free system to 7-8% for wadsleyite containing 1.5 wt.% H₂O. Thus, the magnitude of the seismic velocity change at 410-km depth can be expected to be sensitive to the presence of H₂O in olivine polymorphs.