Single-crystal elasticity of wadsleyites, β -Mg2SiO4, containing 0.37–1.66 wt.% H2O

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, KS0, and shear modulus, G0, were calculated using VRH (Voigt–Reuss–Hill) averages. The results are: KS0 = 165.4(9) GPa, G0 = 108.6(6) GPa for wadsleyite with 0.37 wt.% H2O; KS0 = 160.3(7) GPa, G0 = 105.3(6) GPa for 0.84 wt.% H2O; KS0 = 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): KS0 = 170.9(9) − 13.0(8) CH2O, G0 = 111.7(6) − 7.8(4) CH2O, where CH2O 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.