Electrical conductivity of NaCl-H₂O fluid in the crust

Ionic electrical conductivity of NaCl-H₂O fluid as a function of pressure (0.2-2.0 GPa), temperature (673-2000 K), and NaCl concentration (0.6-9.6 wt %) was investigated using molecular dynamics (MD) simulations. Conductivity versus NaCl concentration has a nonlinear relationship due to the presence of electrically neutral ion pairs in concentrated solutions. The calculated conductivity at 0.6 wt % NaCl is consistent with the available experimental data, and the calculated conductivity at higher temperatures shows a greater degree of pressure dependence. The major factors controlling the conductivity are the density of the NaCl-H₂O fluid and the permittivity of solvent H₂O. A purely empirical equation for deriving the conductivity was proposed. Highly conductive zones below a depth of 35 km in the middle portion of the continental crust can be interpreted by the presence of NaCl-H₂O fluid with the salinity ranging from 0.2 to 7.0 wt %. A highly conductive zone observed at a depth of 20 to 40 km above the subducting oceanic crust in Cascadia can be explained by the presence of low-salinity (0.5 wt %) NaCl-H₂O fluid possibly generated by the dehydration of basalt.