BRINE Distribution in Harite Rocks-Inference from Measured Electrical CONDUCTIVITY-

Intercrystalline fluid can significantly affect rheological and transport properties of rocks. Its influences are strongly depended on its distribution. The dihedral angle between solid and liquid phases has been widely accepted as a key parameter that controls solid-liquid textures. The liquid phase is not expected to be interconnected if the dihedral angle is larger than 60°. However, observations contradictory to dihedral angle values have been reported. Watanabe (2010) suggested that the grain boundary fluid coexist with a positive dihedral angle. Similar thin fluid films might exist in grain boundaries of crustal rocks, and play important roles in crustal processes. In order to understand the nature of this fluid, we have studied the distribution of brine in halite rocks through measurements of conductivity. A sample was prepared by cold-pressing (the axial stress of 140MPa, 40minutes) and annealing (the temperature of 160°C and 180MPa confining pressure for about 160hours) of wet NaCl powder. During annealing, conductivity of a sample was monitored (2-electrode method) to reach a quasi-stationary value. The volume fraction of water is estimated to be 0.1×0.01% based on the FTIR measurement. Grains are polygonal and their diameters are ~100 μm. The cylindrical (D=9mm, L=3mm) surface of an annealed sample was weakly dried and coated with RTV rubber to suppress the conduction on the surface. Conductivity measurements were made at various pressure and temperature conditions(P<180 MPa, T<180°C). The dihedral angle of NaCl-water system is 66° at P=30MPa and T=30°C, and it becomes smaller than 60° at high pressure and temperature conditions(Holness & Lewis, 1997). Measured conductivity was of the order of 10^-7~10^-9 (S/m). When the temperature was fixed, a lower conductivity was observed for a higher pressure. The decrease in conductivity was typically 50% for the increase in pressure of 30MPa. For a given pressure, the conductivity increased with increasing pressure. The conduction was dominated by the conduction through the fluid over a whole range of pressure and temperature (30<P<180 MPa, 30<T<180°C). The conductivity was higher than of dry halite by 1-2 orders of magnitude. It changes quickly in response to the change in pressure. The dominant conduction paths were not triple-junction tubes. A triple-junction tube is so stiff that it can't give observed conductivity changes in response to changes in pressure. No remarkable change in conductivity was observed around the condition of the dihedral of 60°. Although the interconnection of triple-junction tubes might drastically change at the dihedral angle of 60°, its influence on the bulk conductivity is masked by more conductive paths. Grain boundary brine must be the dominant conduction paths. Using the physical properties of water, we evaluated the resistance of a water-filled tube with an elliptical cross-section. It can't give observed pressure dependence of conductivity. We thus suggest that thin water films exist in grain boundaries. In thin water films, water molecules can't move freely under the influence of solid surfaces. Transport properties in a thin water film will depend on its thickness. The pressure dependence of conductivity may be attribute to the nature of thin water film.