Velocity crossover between hydrous and anhydrous forsterite at high pressures
Abstract
The elastic properties of hydrous forsterite, Mg_{2  x}SiO_{4}H_{2x}, are relevant to interpreting seismic velocity anomalies in the Earth's mantle. In this study, we used Brillouin scattering to determine the singlecrystal elasticity of forsterite with 0.9(1) wt.% H_{2}O (x = 0.14) to 14 GPa. 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 (G_{0}′ = 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 300 K. 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 ∼ 34 GPa result in higher hydrous forsterite velocities at pressures corresponding to depths below ∼ 120 km. At the pressure of the 410km 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 410km 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 ∼ 0.4 wt.% H_{2}O in forsterite (at 60 mol%) could lower the P and S velocity contrast at 410km 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.
 Publication:

Earth and Planetary Science Letters
 Pub Date:
 May 2010
 DOI:
 10.1016/j.epsl.2010.02.025
 Bibcode:
 2010E&PSL.293..250M