Partial‐melt electrical conductivity: Influence of melt composition

Abstract

The electrical conductivity of a partial melt is influenced by many factors, including melt conductivity, crystalline conductivity, and melt fraction, each of which is influenced by temperature. We have performed measurements of bulk conductivity as a function of temperature of an Fo 80 ‐basalt partial melt between 684° and 1244°C at controlled oxygen fugacity. Melt fraction and composition variations with temperature calculated using MELTS [Ghiorso and Sack, 1995] indicate that the effect on melt conductivity of changing melt composition is balanced by changes in temperature (T). Thus bulk conductivity as a function of T or melt fraction in this system can be calculated assuming a constant melt conductivity. The bulk conductivity is well modeled by simple parallel calculations, by the Hashin‐Shtrikman upper bound, or by Archie's law (σ partial melt /σ melt = C 1 X m n ). We estimate apparent values of the Archie's law parameters between 1150° and 1244°C as C 1 = 0.73 ± 0.02 and n = 0.98 ±0.01. Estimates of the permeability of the system are obtained by using an electrical conductivity‐critical scale length relationship and range from ∼10 −14 to 10 −18 m 2 , comparing favorably with previously published values.