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 Fo(80)-basalt part ial melt between 684 degrees and 1244 degrees C at controlled oxygen fugaci ty. Melt fraction and composition variations with temperature calculated us ing MELTS [Ghiorso and Sack, 1995] indicate that the effect on melt conduct ivity of changing melt composition is balanced by changes in temperature (T ). Thus bulk conductivity as a function of T or melt fraction in this syste m can be calculated assuming a constant melt conductivity. The bulk conduct ivity is well modeled by simple parallel calculations, by the Hashin-Shtrik man upper bound, or by Archie's law (sigma(partial) (melt)/sigma(melt) = C1 Xmn). We estimate apparent values of the Archie's law parameters between 11 50 degrees and 1244 degrees 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 elect rical conductivity-critical scale length relationship and range from simila r to 10(-14) to 10(-18) m(2), comparing favorably with previously published values.