Single-crystal elasticity of wadsleyites, β -Mg 2SiO 4, containing 0.37–1.66 wt.% H 2O

Abstract

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.% H 2O 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, G 0, were calculated using VRH (Voigt–Reuss–Hill) averages. The results are: K S0 = 165.4(9) GPa, G 0 = 108.6(6) GPa for wadsleyite with 0.37 wt.% H 2O; K S0 = 160.3(7) GPa, G 0 = 105.3(6) GPa for 0.84 wt.% H 2O; K S0 = 149.2(6) GPa, G 0 = 98.6(4) GPa for 1.66 wt.% H 2O. 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 H 2O , G 0 = 111.7(6) − 7.8(4) C H 2O , where C H 2O is the H 2O weight percentage. Compared with anhydrous wadsleyite, addition of 1 wt.% H 2O 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 H 2O 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 H 2O-free system to 7–8% for wadsleyite containing 1.5 wt.% H 2O. Thus, the magnitude of the seismic velocity change at 410-km depth can be expected to be sensitive to the presence of H 2O in olivine polymorphs.