1. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA

Correspondence to: James A. Tyburczy¹ Correspondence and requests for materials should be addressed to J.A.T. (Email: jim.tyburczy@asu.edu).

A deep-seated melt or fluid layer on top of the 410-km-deep seismic discontinuity in Earth's upper mantle, as proposed in the transition-zone 'water filter' hypothesis¹, may have significant bearing on mantle dynamics and chemical differentiation. The geophysical detection of such a layer has, however, proved difficult. Magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt. Here we use these methods to search for a distinct structure near 410-km depth. We calculate one-dimensional forward models of the response of electrical conductivity depth profiles, based on mineral physics studies of the effect of incorporating hydrogen in upper-mantle and transition-zone minerals. These models indicate that a melt layer at 410-km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern United States (Tucson)². The 410-km-deep melt layer in this model has a conductance of 3.0  10⁴ S and an estimated thickness of 5–30 km. This is the only regional data set that we have examined for which such a melt layer structure was found, consistent with regional seismic studies³. We infer that the hypothesized transition-zone water filter¹ occurs regionally, but that such a layer is unlikely to be a global feature.

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