Electrical conductivity of NaCl-bearing aqueous fluids to 600 °C and 1 GPa

Abstract

The electrical conductivity of aqueous fluids containing 0.01, 0.1, and 1 M NaCl was measured in an externally heated diamond cell to 600 °C and 1 GPa. These measurements therefore more than double the pressure range of previous data and extend it to higher NaCl concentrations relevant for crustal and mantle fluids. Electrical conductivity was generally found to increase with pressure and fluid salinity. The conductivity increase observed upon variation of NaCl concentration from 0.1 to 1 M was smaller than from 0.01 to 0.1 M, which reflects the reduced degree of dissociation at high NaCl concentration. Measured conductivities can be reproduced (R 2 = 0.96) by a numerical model with log $$\sigma$$ = −1.7060– 93.78/T + 0.8075 log c + 3.0781 log $$\rho$$ + log $$\varLambda$$ 0(T, $$\rho$$ ), where $$\sigma$$ is the conductivity in S m−1, T is temperature in K, c is NaCl concentration in wt%, $$\rho$$ is the density of pure water (in g/cm3) at given pressure and temperature, and $$\varLambda$$ 0 (T, $$\rho$$ ) is the molar conductivity of NaCl in water at infinite dilution (in S cm2 mol−1), $$\varLambda$$ 0 = 1573–1212 $$\rho$$ + 537 062/T–208 122 721/T 2. This model allows accurate predictions of the conductivity of saline fluids throughout most of the crust and upper mantle; it should not be used at temperatures below 100 °C. In general, the data show that already a very small fraction of NaCl-bearing aqueous fluid in the deep crust is sufficient to enhance bulk conductivities to values that would be expected for a high degree of partial melting. Accordingly, aqueous fluids may be distinguished from hydrous melts by comparing magnetotelluric and seismic data. H2O–NaCl fluids may enhance electrical conductivities in the deep crust with little disturbance of v p or v p/v s ratios. However, at the high temperatures in the mantle wedge above subduction zones, the conductivity of hydrous basaltic melts and saline aqueous fluids is rather similar, so that distinguishing these two phases from conductivity data alone is difficult. Observed conductivities in forearc regions, where temperatures are too low to allow melting, may be accounted for by not more than 1 wt% of an aqueous fluid with 5 wt% NaCl, if this fluid forms a continuous film or fills interconnected tubes.