Electrical conductivity of NaCl-H2O fluid in the crust

摘要

Ionic electrical conductivity of NaCl-H2O fluid as a function of pressure (0.2-2.0GPa), temperature (673-2000K), and NaCl concentration (0.6-9.6wt%) 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 0.6wt% 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 35km 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 7.0wt%. A highly conductive zone observed at a depth of 20 to 40km above the subducting oceanic crust in Cascadia can be explained by the presence of low-salinity (0.5wt%) NaCl-H2O fluid possibly generated by the dehydration of basalt.