Inference on Water Content in the Mantle Transition Zone Near Subducted Slabs From Anisotropy Tomography

Abstract

We examine the patterns of radial anisotropy in global tomography images of the mantle transition zone near subducted slabs in the western Pacific. Fast SV velocity anomalies are observed in this region, which are compatible with anisotropy due to lattice-preferred orientation in wadsleyite. Using mineral physics reports of the dependency of the strength of radial anisotropy on water content in wadsleyite, we estimate the water content in the transition zone near subducted slabs from the tomography images. We find that fast SV anisotropy anomalies over ~1.5% observed beneath subduction zones in the western Pacific are compatible with a low water content (smaller than ~3,000 ppm H/SI), notably beneath the Tonga-Kermadec trenches, the Philippines, and the Sumatra trench.