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 the frequency range 10(-1) to 10(5) Hz at temperatures between 600 and 1200 degrees C at normal pressure and different oxygen fugacities in the stability field of magnetite. The impedance spectroscopic (IS) measurements are compared with the results of melting experiments performed under the same experimental conditions and with the same rock sample as the IS measurements. A strong increase in the electrical conductivity of about one and a half orders of magnitude is observed during partial melting. This increase is explained by the formation of an interconnected network of melt. The wetting of the grain surfaces starts slightly above the solidus. However, complete interconnection of the network is detected at higher temperatures (1070 degrees C) and hence higher melt fractions (10 vol%). This is explained by deviations from the equilibrium fabric of the partially molten sample. With a modified brick layer model (MBLM) the electrical conductivity is calculated as a function of the observed melt portion. This model is valid if the melt forms a complete interconnected network at the grain boundaries. The electrical 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 where partially molten rocks are assumed to exist. The same MBLM is used to estimate the melt fraction that is necessary to explain the high conducting zones (HCZs) observed in the crust beneath the Andean orogen, the Pyrenees and the Tibetan Plateau. At least 14, 4 and 4 vol% of interconnected melt are necessary to produce the observed conductivity, respectively. The actual amount of melt should be higher due to perturbation problems and melt trapped in pockets. (C) 2000 Elsevier Science B.V. All rights reserved.