Mid-crust fluid and water-rock interaction kinetic experiments and their geophysical significance: 1. Basalt and pyroxene in water at high temperatures up to 450°C

The water-basaltic rock interaction (or pyroxene-water ) kinetic experiments are carried out using flow through a packed bed reactor (PBR) and a mixed flow reactor(MFR) in the temperature range(T) from 20 to 450°C and at 23-34MPa. The experimental apparatus consisted of a titanium vessel, a liquid pump, a backpressure regulator, an electrical conductivity detector and a computer for data acquisition and monitoring. The basaltic rock and pyroxene were collected from natural volcanic area, Anhui Province, China. Rock or mineral sample was crushed and sieved to 20-40 mesh and cleaned. The surface area of representative basalt samples is 9.978 m²/g, for pyroxene it is 1.987 m² /g (BET method). As using PBR system, rock samples (10. 2526g) were placed in the vessel. De-ionized and degassed (DDW) water was passed through the rock, or mineral grains at flow rates of 0.5-8 ml/min. As using MFR system mineral and rock sample of 5 grams was put in the vessel. Here, the measured release rates for each metal of the rock are the sum of release rates of the metal in various minerals of the rock. Steady-state kinetics is defined as conditions where dissolution rates are time independent. For water-basalt interaction, the dissolution rates (dis.r.) of Ca, Mg, Fe, Al, Na, K and Si vary with T from 25 to 435°C(or to 550°C). The dis.r. of Si, r_Si increase with T from 25°C to 300°C, and then decrease from 300°C to 435°C (to 550°C). Maximum dis.r. for Si, r_{Si, Mx} is reached at 300°C(or 300 to 400°C, using MFR). The maximum dis.r. for various metals occurred at different T, e.g. r_{Al, Mx} and r_{Na, Mx} are nearly at 350°C, r_{K, Mx} is at 300°C, r_{Ca, Mx} is at 100°C and r_{Mg, Mx} is at 20°C. As T increase above 400°C, dis.r. of Ca, Mg and Fe decrease to very small. The experiments for pyroxene (diopside, hedenbergite)in water indicated that the dis.r. of Ca, Mg, Fe, Al, K and Si also vary with T. The r_Si increase with T from 25 to 300°C, and then decrease with T from 300 to 400 °C. The r_{Si, Mx} occurred at 300°C. r_{Al, Mx} is at 374°C and r_{K, Mx} is near at 350°C, r_{Ca, Mx} and r_{Fe, Mx} are at 200°C and r_{Mg, Mx} is at 100°C. The upper-middle crust is usually at T range from 300 to 450°C, where, high conductivity zone occurs. Geophysical survey indicated that plate collision are ultimately responsible for inducing horizontal faults, detachments (cracking), generating porosity, decreasing pressure, and moving fluids through continents. These processes probably lead to the migration of aqueous fluid in the mid-crust accompanying pressure lowering (possible close to the critical state of aqueous solution) at 300 to 450°C. Therefore, strong water rock interactions occur in mid-crust, cause strong leaching of Si, breakage of silicate framework and rock collapse. Those reactions will further lead to increase rock porosity and drive fluid flow. Those events take important role in enhancing the electric conductivity of rocks in the mid-crust. Simultaneously, water and NaCl-H₂O at 350-450°C in the mid crust have high electric conductivity. Key words: chemical kinetics, critical state, basalt-water interaction, electric conductance, high conductivity zone, high temperature experiment.