Abstract
AbstractWe 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.
Highlights
The mantle transition zone (MTZ) has been suggested to play a key role in the mixing between the upper and lower mantle and in the cycling of fluids and volatiles in the Earth's deep interior (e.g., Hirschmann, 2006)
We examine the patterns of radial anisotropy in global tomography images of the mantle transition zone near subducted slabs in the western Pacific
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, notably beneath the Tonga‐Kermadec trenches, the Philippines, and the Sumatra trench
Summary
The mantle transition zone (MTZ) has been suggested to play a key role in the mixing between the upper and lower mantle and in the cycling of fluids and volatiles in the Earth's deep interior (e.g., Hirschmann, 2006). Some early seismic tomography studies have highlighted that the MTZ is anisotropic (e.g., Beghein & Trampert, 2003; Montagner & Kennett, 1996; Visser et al, 2008), but others showed little or no anisotropy in this region (e.g., Beghein et al, 2006; Panning et al, 2010) or suggested that the observed radial anisotropy is not robust (e.g., Kustowski et al, 2008; Moulik & Ekström, 2014). We perform a series of robustness tests of the observed anisotropy features in the MTZ, which enables their physical interpretation in terms of LPO of the dominant minerals in the MTZ, notably wadsleyite. The water content of the MTZ is inferred from the strength of the observed radial anisotropy using experimental results from mineral physics on wadsleyite
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