Mid-crust fluid and water-rock interaction kinetic experiments and their geophysical significance: 3. in situ measurements of electric conductance of the water-rock interaction system at high temperatures

Recently, we designed a new experimental apparatus, which was used to measure dissolution rates as water-rock interactions, and simultaneously in situ measure the electric conductance of the multi phase (rock - fluid) system. At first, a tubular reactor is put in horizontally. The rock sample was crushed and sieved to 20-40 mesh, and cleaned, then put in a titanium network bag inside of the vessel. The electric conductivity detectors were connected to the two ends of the vessel. Fluid inlet and outlet were also fabricated in the two ends of the vessel. And the furnace, temperature controller, liquid pump, back pressure regulator etc. are involved in the whole experimental system. High temperature (T) and pressure (P) electric conductivity measuring system consists of an electric conductivity detector (ECD limit.), which was reformed by us and connected to the vessel; and a transfer: T23-CDH-UM: 5.67(L)×3.50(W)×5.67(H) (inch).The distance between the two electrode of the sensor is 10cm. The electrode is 5cm of length and its diameter is 5/16 inch. Water-rock interaction experiments were performed using this apparatus. The volume inside of vessel is 10.81 ml, l= 170mm, d=4.5mm and 7.246g rock sample put in the vessel. The fluid velocity was changed from 1.5 to 3.5 ml/min, allowing the water through the sample. As in situ to measure the electric conductance of the rock-fluids in the vessel, continuously record the electric conductance, each record in 5 seconds. Water-basalt interaction experiments were carried out and in situ measured electric conductance at high T up to 450°C and at 22 to 36MPa. Basalt sample was collected from natural outcrop (volcanic area in Yangtze valley, China, which is K-rich trachy-basalt. Rock sample was crushed and sieved to 20-40 mesh, and cleaned. 8.0097g sample was put in the vessel (surface area: 1.37m²/g ). Experiments found dissolution rates (dis.r.) for different metals of the rock vary with T. Usually, dis.r., r_Si increase with T from 20 to 400°C, then decrease with continued increasing T. At 24MPa, the r_Si reach the maximum at 400°C (r_{Si, Mx}= 55.41 mol.min^-1 m^-210^-5). The maximum dissolution rates for others at different T: r_{Al, Mx} at 397°C(=4.9 mol.min^-1m^-210^-5), r_{K, Mx} at 308°C(=18.37 mol.min^-1m^-210^-5), r_{Na, Mx} at 400°C(=5.49 mol.min^-1m^-210^-5), r_{Ca, Mx} at 210°C (=2.44 mol.min^-1m^-210^-5), r_{Mg, Mx} at 109°C, r_{Fe, Mx} at 398°C. The maximum electric conductance is 3.3ms/cm at 300°C, and 1.7ms/cm at 400°C. As water basalt interaction experiment at 34MPa, r_{Si, Mx}= 54.62 mol.min^-1m^-210^-5 at 400°C. Simultaneously, maximum electric conductance 3.4ms at 400°C. Those experiments demonstrated that water-rock interaction is very strong at the T range from 300 to 400°C, and the rock-fluid system exhibits a high electric conductivity. Furthermore, those experiments can be used to illustrate geophysical feature of the mid crust, where high electric conductivity zone appears. For the future, we will change fluid and rock feature to simulate the water-rock interaction in the mid-crust. Key words: chemical kinetics, critical state, water-rock interaction, electric conductance, high conductivity zone, high temperature experiment.