Effect of hydration on the single-crystal elasticity of Fe-bearing wadsleyite to 12 GPa

The single-crystal elastic properties of Fe-bearing wadsleyite with 1.93 wt% H₂O (Mg_1.634Fe_0.202H _0.305SiO₄) have been determined by Brillouin scattering. At ambient conditions, the aggregate bulk and shear moduli (K_S0, G₀) of this wadsleyite are 156.2(5) and 98.0(3) GPa, respectively. Compared to the corresponding anhydrous wadsleyite, 1.93 wt% H₂O lowers K_S0 and G0 by 8.1% and 9.3%, respectively. High-pressure measurements up to 12 GPa show that the pressure derivative of the bulk modulus, K'_S0 = 4.8(1), is similar to that of the anhydrous Fe-wadsleyite with reported values of 4.6-4.74, but the addition of H₂O increases the pressure derivative of the shear modulus, G'₀ from 1.5(1) to 1.9(1). This contrasts with the G'₀ of Fe-free wadsleyite, which is the same within uncertainty for the hydrous and anhydrous phases. As a result, both the compressional- and shear-wave velocities (vP, vS) of hydrous Fe-bearing wadsleyite are about 200(±24) m/s slower than anhydrous Fe-bearing wadsleyite at transition zone pressures.