Velocity crossover between hydrous and anhydrous forsterite at high pressures

The elastic properties of hydrous forsterite, Mg_2-xSiO₄H_2x, are relevant to interpreting seismic velocity anomalies in the Earth's mantle. In this study, we used Brillouin scattering to determine the single-crystal elasticity of forsterite with 0.9(1) wt.% H₂O (x=0.14) to 14GPa. Aggregate bulk and shear moduli of hydrous forsterite increase with pressure at a greater rate than those of the corresponding anhydrous phase. Compared with anhydrous forsterite, we observe a 7% increase in the pressure derivative of the bulk modulus (K_S0-=4.50(5)), and a 25% increase in the pressure derivative of the shear modulus (G₀-=1.75(5)) for forsterite with near maximum possible water content. Using our results, we calculated the compressional, V_P, and shear, V_S, velocities of forsterite as a function of pressure at 300K. Whereas V_P and V_S of hydrous forsterite are 0.6% and 0.4% slower than those of anhydrous forsterite at ambient pressure, velocity crossovers at ~3-4GPa result in higher hydrous forsterite velocities at pressures corresponding to depths below ~120km. At the pressure of the 410-km discontinuity, V_P and V_S of hydrous forsterite exceed those of anhydrous forsterite by 1.1(1)% and 1.9(1)%, respectively. This implies that incorporation of water could decrease the magnitude of the velocity contrast at 410-km depth between forsterite and wadsleyite. Although the effects of hydration on temperature derivatives of the elastic moduli of forsterite and wadsleyite are not yet known, from the current data we estimate that the presence of ~0.4wt.% H₂O in forsterite (at 60mol%) could lower the P and S velocity contrast at 410-km depth to 3.8(4)% and4.8(6)%, respectively. At high pressures, hydration also decreases the V_P/V_S ratio of forsterite, and lowers the maximum P wave azimuthal anisotropy and S wave splitting of forsterite.