Electrical conductivity beneath Iceland - constraints imposed by magnetotelluric results on temperature, partial melt, crust- and mantle structure
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Magnetotelluric results from 38 sites in North Iceland yield a detailed spatial distribution of conductivity versus depth. A high-conductivity layer, about 10 Ωm, marking the transition between crust and upper mantle is detected throughout the surveyed area, its depth increases from 10 km within the zones of present tectonic activity to 20–30 km at a 50–100 km distance from the rift axis. The high conductivity layer indicates some basaltic melt fraction at temperatures around 1,000°C. The calculated average crustal temperature gradients are 100–150°C/km at the rift axis and 50–70°C/km at about 100 km distance. Below this anomalous zone, down to 150 km depth, the conductivity decreases to 100 Ωm 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 °C/km. However, a highly conducting asthenosphere exists below 150 km depth. The following petrological model for Iceland is suggested: Diapiric updoming of the asthenosphere leads to partial fusion of parent ultra basic mantle material, resulting in segregation and upward migration of basaltic melt. Lower density, lower viscosity and higher mobility relative to the solid phase causes the melt to rise faster than the ultrabasic material. This melt fraction then forms the basaltic crust of Iceland which is thickening away from the ridge axis with increasing age.
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