Partial-melt electrical conductivity: Influence of melt composition

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 Fo₈₀-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 = C₁X_mⁿ). We estimate apparent values of the Archie's law parameters between 1150° and 1244°C as C₁ = 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 m², comparing favorably with previously published values.