APPLICATION OF ISOTOPIC DOPING TECHNIQUES TO EVALUATION OF REACTION-KINETICS AND FLUID MINERAL DISTRIBUTION COEFFICIENTS - AN EXPERIMENTAL-STUDY OF CALCITE AT ELEVATED-TEMPERATURES AND PRESSURES

Isotopic doping techniques have been combined with flexible cell hydrothermal equipment to evaluate the kinetics of calcite recrystallization at elevated temperatures and pressures. Two "free-drift" experiments were performed with a fluid/calcite mass ratio of 15, using Iceland spar calcite as the starting solid and a 0.5 m NaCl fluid of variable pH and total dissolved carbon. The first experiment was conducted at 250-degrees-C and 500 bar with a starting fluid near calcite saturation, while a second experiment was conducted at a series of temperature steps of 300-degrees, 350-degrees and 400-degrees-C and 500 bar, with a fluid chemistry initially adjusted to be far from calcite saturation. As determined from the changes in fluid isotopic compositions, the rates of calcite recrystallization were observed to increase with increasing temperature, but were found to be exceedingly slow under near-equilibrium conditions. Results indicate an initial period of relatively fast dissolution, followed by a period of slow-steady-state recrystallization for which rates of 10(-12.7), 10(-12.33), 10(-11.55) and 10(-10.88) (mol s-1 cm-2) were measured at 250-degrees, 300-degrees, 350-degrees and 400-degrees-C, respectively, yielding an activation energy of 25.5 kcal mol-1 for this process. Using these values, estimates of the amount of time required to approach isotopic equilibrium between a pore fluid and a host carbonate rock are calculated to be on the order of several days to several million years, depending on the temperature, fluid flow rate, and the relative concentrations of the element in the fluid and host rock. Fluid/calcite distribution coefficients for Sr were determined in the experiment performed at 250-degrees-C and 500 bar, and were found to be dependent on the rate of calcite recrystallization, corroborating previous reports of this type of behavior.