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

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. Plain Language Summary Tectonic plates plunge into the mantle at trenches, carrying water from the oceans. Some of this water may go down to the mantle transition zone between 410‐ and 660‐km depth, where minerals have a large water storage capacity. In this study, we use observations of seismic anisotropy, the directional dependency of seismic wave speed, which is sensitive to the water content in the mantle transition zone. We find that the mantle transition zone beneath some subduction zones is drier than previously thought. Key Points Fast SV velocity anomalies are observed in the upper part of the transition zone near subducted slabs in the western Pacific The observed anomalies are compatible with anisotropy due to lattice‐preferred orientation in wadsleyite The fast SV velocity anomalies may imply a low water content, notably beneath the Tonga‐Kermadec, the Philippines, and the Sumatra trenches