Influence of NaCl Aqueous Solutions on Isotopic Equilibria and Rates of Exchange in Mineral-Fluid Systems

Abstract

The effect of NaC1 aqueous solutions on the rates of isotopic exchange and fractionation factors between minerals and fluids has been evaluated at elevated temperatures using the available experimental data. Rates of oxygen isotopic exchange are significantly increase by interaction of a solid with NaC1 solutions. For examples, the rate of granite H2O oxygen isotopic exchange is 10/-9.45 moles 0 m/-2 sec -1 at 200°C, but 10/-7.75 for granite -0.5 m NaC1 interaction. The durations required to attain 90% isotopic exchange in the granite fluid system have been calculated using a modified surface exchange rate model. These calculations indicate that granite pure water interaction requires approximately 3700 years to reach 90% exchange but only 74 years for reaction with 0.5 m NaC1 at 200°C. Isotopic fractionation factors between minerals and aqueous NaC1 solutions can differ significantly from the mineral pure water values. The salt effect can be as large as 2 to 3.5 permil for oxygen and 12 permil for hydrogen at temperatures above 150°C. If ignored, the salt effect associated with 4 m NaC1 solution can lead to maximum error in geothermometers estimates for oxygen of 35°C in the quartz fluid system, and 50°C for hydrogen in the kaolinite fluid system. The magnitude of the salinity effects is far too great to be ignored, and must be accounted for when modeling isotopic exchange in hydrothermal system.