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Electromagnetic detection of a 410-km-deep melt layer in the southwestern United States


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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Figure 1: Forward modelled MT response of upper-mantle conductivity depth profile.
Figure 2: Electrical conductivity of various melts as a function of inverse temperature.
Figure 3: Forward modelling results for the southern Basin and Range (Tucson) data set2.


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We thank S. Constable and E. Garnero for comments and discussion. This work was supported by the National Science Foundation.

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Correspondence to James A. Tyburczy.

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Toffelmier, D., Tyburczy, J. Electromagnetic detection of a 410-km-deep melt layer in the southwestern United States. Nature 447, 991–994 (2007).

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