From: Evans et al., Asymmetric electrical structure in the mantle beneath the East Pacific Rise at 17°S, Science, 286, 752-756, 1999

This figure shows two models of the electrical conductivity of the mantle in the MELT area. The most consistent and robust feature is the abrupt transition at the axis from (relatively) conductive mantle to the west to electrically resistive mantle, consistent with dry olivine with virtually no interconnected melt, to the east.