Electrical conductivity of the continental crust

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Abstract
Geophysical measurements indicate that the Earth's continental lower crust has a high electrical conductivity for which no simple cause has been found. Explanation usually relies on either saline fluids saturating the pores, or interconnected highly conducting minerals such as graphite, Fe/Ti oxides and sulphides, providing conducting pathways. Attempts in the laboratory to clarify the problem have, hitherto, been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270°C and 1000°C. Here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids. Our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism(s). We have used the experimental data to construct electrical conductivity/depth profiles for the continental crust, which, when compared with profiles obtained from magnetotelluric observations, demonstrate that a mid to lower crust composed of amphibolite saturated with 0.5 M NaCl shows electrical conductivities sufficient to explain conductivity/depth profiles for the continental crust inferred from geophysical measurements.