Forward Models of the Electromagnetic Response of Upper Mantle Structure: Models with a 410 km Deep Upper Mantle Melt Layer

We present mineral-physics based forward models of the electromagnetic response of regional upper mantle structure focusing on the possibility of the presence of a melt or fluid zone at 410 km depth. Laboratory data are used to construct 1-D electrical conductivity-depth models for which the water-content of the upper mantle (olivine) and transition zone (wadsleyite and ringwoodite) are the only variables. These models are used to fit regional MT/GDS results (phi (tau) and rho (tau)). If the estimated transition zone water content is greater than a threshold value of 0.4 wt per cent water, then a thin high conductivity (melt) zone (based on estimates of melt conductivity at depth) is added to the model at 410 km depth. For the Southern Basin and Range (Tucson) results of Egbert et al. (1992), the transition zone water content threshold is exceeded and a significant improvement in the fit (reduction in chi-squared) results from inserting a high conductivity zone. The Southern Basin and Range model incorporates approximately 0.08 wt per cent H2O in the upper mantle (to approximately 400 km depth), 0.6 wt per cent H2O in the transition zone (consistent with upper mantle H2O solubility and melting considerations) and a 410 km deep, 5-30 km thick high-conductivity (melt) layer with a conductance of 30,000 S. This result is consistent with teleseismic observations of a 410 km deep low velocity zone in the western U.S. (Song et al.,, 2004). For other regions, such as the Canadian Shield (Schultz et al., 1993) or Europe (Olson 1999), the transition zone water-content does not exceed the threshold value and an explicit improvement in fit does not result from the addition of a high conductivity zone.