Electrical conductivity measurements of aqueous fluids under crustal conditions in multi-anvil apparatus

The electrical conductivity of aqueous fluids is critical for interpreting the electrical anomalies observed by magnetotellurics in the Earth's crust. Conductivity measurements of fluids at high temperature and pressure were mainly conducted in cold-sealed pressure vessels, hydrothermal diamond anvil cells, and piston-cylinder apparatus, by using precious metals, such as Pt-Rh or Au-Pd as sample capsules. However, this research has never been performed in a multi-anvil apparatus. In addition, metal capsules employed in previous studies were costly and difficult to manufacture mechanically in the laboratory. In this study, we designed a novel, economical, chemical inertness, and tractable sample capsule made of polytetrafluoroethylene (PTFE), and by using this capsule, we successfully measured the conductivity of NaCl solutions in a multi-anvil apparatus under conditions of 323-598 K and 0.5-1.0 GPa. Our results are consistent with those from diamond anvil cells and piston-cylinder apparatus. Besides being used in conductivity measurement, the new and low-cost method has potential applications for investigating other physicochemical properties, such as rock-water interactions and fluid sound velocity under shallow crustal conditions. Furthermore, higher temperature and pressure can be realized by replacing the PTFE components with metallic materials, enabling more extensive exploration of fluid behavior in the Earth's deep interior.