Abstract:
We have developed a new method for determining equilibrium hydrogen isotope fractionation factors between hydrous minerals and water. In contrast to traditional experimental methods that use finely powdered minerals as starting material and analyze the experimental run products with bulk analytical techniques, we use large single crystals as starting material and then analyze the outer 0.5-2.0 μm of the isotopically exchanged crystals by ion microprobe. Although providing somewhat less precise analytical data than can be obtained with conventional methods, the advantage of the present technique is that it allows fractionations to be determined in experiments in which isotopic exchange demonstrably occurs through a diffusional process and not by recrystallization. This, in turn, permits acquisition of unambiguously reversed experimental data on equilibrium fractionation factors. The technique was applied to the epidote-water system for which we report fractionation data between 300-600°C determined at a pressure of about 2.1 kbar. Despite initial epidote-water fractionations that were far from equilibrium in some experiments, diffusional exchange rates were sufficiently high in this system to bracket the equilibrium fractionation within analytical error to temperatures as low as 300°C. The measured fractionations vary systematically with temperature, decreasing from -36%% at 300°C to -54%% at 600°C. Within the experimental temperature range, these fractionations can be represented by the equation 1000 ln α (epidote-water) = 9.3 x 106/T2 - 61.9. The present method should be applicable to other mineral-water systems and help to resolve some of the existing discrepancies with regard to the magnitude and temperature dependence of hydrogen isotope fractionation factors.