Electrical conductivity of dry lower crustal rocks

This study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings. We present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 degrees C to 1000 degrees C. We observe statistically significant differences between rock types: mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group, aphanitic (fine-grained) rocks have higher conductivity than phaneritic (coarse-grained) ones. 'Best-fitting' curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity. Because the laboratory rocks are dry, their conductivities are lower at a given temperature than they would be if fluids or volatiles were present; hence any temperatures inferred from magnetotelluric (MT) contivities are upper bounds.