Effect of structural water on the elasticity of orthopyroxene

As a major nominally anhydrous mineral (NAM) in the Earth's upper mantle, orthopyroxene could host up to several hundred parts per million H2O in its crystal structure and transport the H2O to the deep Earth. To study the effect of structural H2O on the elasticity of orthopyroxene, we have measured the single-crystal elasticity of Mg1.991Al0.065Si1.951O6 with 842-900 ppm H2O and 1.64 +/- 0.20 wt% Al2O3 at ambient conditions using Brillouin spectroscopy. The best-fit single-crystal elastic moduli (C(ij)s), bulk (K(S)0), and shear (G(0)) modulus of the hydrous Al-bearing orthopyroxene were determined as: C-11 = 235(2) GPa, C-22 = 173(2) GPa, C-33 = 222(2) GPa, C-44 = 86(1) GPa, C-55 = 82(1) GPa, C-66 = 82(1) GPa, C-12 = 75(3) GPa, C-13 = 67(2) GPa, and C-23 = 49(2) GPa, K-S0 = 111(2) GPa, and G(0) = 78(1) GPa. Systematic analysis based on the results presented in this and previous studies suggests that the incorporation of 842-900 ppm H2O would increase C-13 by 12.0(7)% and decrease C-23 by 8.6(8)%. The effects on C-11, C-22, C-33, C-44, C-66, K(S)0, and V-P are subtle if not negligible when considering the uncertainties. The C-55, C-12, G(0), and V-S are not affected by the presence of structural H2O. Although laboratory experiments show that Fe,Al-bearing orthopyroxenes can host up to 0.8 wt% H2O in its structure, future high-pressure-temperature elasticity measurements on orthopyroxene with higher H2O content are needed to help better quantify this effect.