3D modelling of electrical anisotropy from electromagnetic array data: hypothesis testing for different upper mantle conduction mechanisms

Upper mantle electrical conductivities provide, besides seismic velocities or thermal diffusivities, another source of information on upper mantle structure and composition. We present the results of a long-period magnetotelluric dataset, consisting of 64 sites covering an area of 300 kin x 400 km in central Germany, mapping electrical conductivities at the base of the lithosphere. To seperate the effect of crustal and upper mantle structures, we interprete the obtained dataset by 31) conductivity modelling. We observe high conductivities and anisotropies at asthenospheric upper mantle depth, with main bulk conductivities of similar to 1-2 S/m in west-east and similar to 4 x 10(-3) S/m in north-south direction, differing by about two orders of magnitude. In comparing with results from laboratory measurements on electrical conductivites for various conduction mechanisms we are able to confine the conduction mechanisms possibly dominating at these depths. With melt volume fractions of 1% only partial melt can be excluded as origin of the high upper mantle conductivities. Instead conductivity enhancement due to hydrogen diffusion along the olivine [100]-axis is favoured as the dominant conduction mechanism at the base of the lithosphere. (c) 2004 Elsevier B.V. All rights reserved.