The influence of partial melting on the electrical behavior of crustal rocks: laboratory examinations, model calculations and geological interpretations

The complex electrical impedance of a granulite has been determined over th e frequency range 10(-1) to 10(5) Hz at temperatures between 600 and 1200 d egrees C at normal pressure and different oxygen fugacities in the stabilit y field of magnetite. The impedance spectroscopic (IS) measurements are com pared with the results of melting experiments performed under the same expe rimental conditions and with the same rock sample as the IS measurements. A strong increase in the electrical conductivity of about one and a half ord ers of magnitude is observed during partial melting. This increase is expla ined by the formation of an interconnected network of melt. The wetting of the grain surfaces starts slightly above the solidus. However, complete int erconnection of the network is detected at higher temperatures (1070 degree s C) and hence higher melt fractions (10 vol%). This is explained by deviat ions from the equilibrium fabric of the partially molten sample. With a mod ified brick layer model (MBLM) the electrical conductivity is calculated as a function of the observed melt portion. This model is valid if the melt f orms a complete interconnected network at the grain boundaries. The electri cal conductivity at different temperatures was modeled using the results of the melting experiments and the MBLM (sigma(melt) = 10 S/m with E-A (melt) = 1 eV; sigma(solid) = 0.01 S/m with E-A (solid) = 1.4 eV). The results of the laboratory examinations were applied to different crustal regions wher e partially molten rocks are assumed to exist. The same MBLM is used to est imate the melt fraction that is necessary to explain the high conducting zo nes (HCZs) observed in the crust beneath the Andean orogen, the Pyrenees an d the Tibetan Plateau. At least 14, 4 and 4 vol% of interconnected melt are necessary to produce the observed conductivity, respectively. The actual a mount of melt should be higher due to perturbation problems and melt trappe d in pockets. (C) 2000 Elsevier Science B.V. All rights reserved.