Anomalous Sound Velocities of Talc at High Pressure and Implications for Estimating Water Content in Mantle Wedge

Abstract

AbstractTalc is widely distributed in metamorphic and hydrothermally altered rocks, and its velocity is useful for constraining the velocity of mantle wedge. In this study, we used ultrasonic interferometry to study the elastic properties of talc polycrystalline samples at high pressure (up to 5.6 GPa). VP increases monotonically with increasing pressure, and VS increases with pressure up to ∼3.2 GPa, but negative pressure dependence occurs above ∼3.2 GPa. Talc has a VP/VS ratio of 1.63–2.14 and Poisson's ratio of 0.20–0.36 at the pressure of 1.2–5.6 GPa, which can be used to explain some anomalous seismic observations in subduction zone. Combining the elastic properties of talc with those of olivine, orthopyroxene, and antigorite, the wave velocity of mantle wedge was calculated as a function of water content. The results suggest that lower water content is needed to match the observed values of seismic tomography when talc is present, compared with the antigorite scenario.