Electrical conductivity of water‐undersaturated crustal melting

Water‐undersaturated melting in the crust can occur at lithostatic pressure in the presence of an H 2 O‐CO 2 fluid, of no CO 2 or fluid but with all H 2 O bound structurally in hydrous minerals, or of an insufficient amount of H 2 O fluid to saturate a melt at liquidus temperatures. The composition of any fluid in equilibrium with possible source rocks depends on the metamorphic grade of the rocks; the fluid at lithostatic pressure in ductile granulite facies rocks would be CO 2 ‐rich, while an increased fraction of H 2 O in a fluid is probable in lithologies of lower metamorphic grade or in rocks of the brittle regime. With limited amounts of pore fluid and hydrous minerals, melting at constant pressure is a highly nonlinear function of temperature and may extend over a broad temperature range. Electrical conductivity of the resultant feldspathic liquids can be estimated to first order from early conductivity measurements on granite under conditions of excess water and recent theories on dissolution of H 2 O in aluminosilicate magmas. Experimental data suggest especially for the deep crust that conductivity of melt in equilibrium with its source residuum falls as melt fraction increases due to dilution of water in spite of increased temperature. At the lesser pressures of high‐level magma chambers, pressure‐temperature‐water content relations of hydrous melt conductivity should be clarified by further laboratory examination. Melt fraction and temperature estimates derived from field electrical surveys are complicated seriously by melt phase tortuosity in the crystalline matrix, by H 2 O content of the source rock, and by the possibility of an H 2 O‐CO 2 fluid.