Electrical conductivity beneath Iceland-constraints imposed by magnetotelluric results on temperature, partial melt, crust- and mantle-structure

Magnetotelluric results from 38 sites in N Iceland yield a detailed spatial distribution of conductivity versus depth. A high-conductivity layer, approx 10OMEGA m, marking the transition between crust and upper mantle is detected throughout the surveyed area, its depth increases from 10km within the zones of present tectonic activity to 20-30km at a 50-100km distance from the rift axis. The high conductivity layer indicates some basaltic melt fraction at temperatures approx 1000oC. Below this anomalous zone, down to 150km depth, the conductivity decreases to 100OMEGA m due to the ultrabasic chemical composition of the upper mantle and a possible decrease of melt content with increasing depth. The temperature gradient within the upper mantle cannot be more than a few oC/km. However, a highly conducting asthenosphere exists below 150km depth. A petrological model for Iceland is suggested.-from Author