A new technique for determining equilibrium hydrogen isotope fractionationfactors using the ion microprobe: Application to the epidote-water system

We have developed a new method for determining equilibrium hydrogen isotope fractionation factors between hydrous minerals and water. In contrast to t raditional experimental methods that use finely powdered minerals as starti ng material and analyze the experimental run products with bulk analytical techniques, we use large single crystals as starting material and then anal yze the outer 0.5-2.0 mu m of the isotopically exchanged crystals by ion mi croprobe. Although providing somewhat less precise analytical data than can be obtained with conventional methods, the advantage of the present techni que is that it allows fractionations to be determined in experiments in whi ch isotopic exchange demonstrably occurs through a diffusional process and not by recrystallization. This, in turn, permits acquisition of unambiguous ly reversed experimental data on equilibrium fractionation factors. The tec hnique was applied to the epidote-water system for which we report fraction ation data between 300-600 degrees C determined at a pressure of about 2.1 kbar. Despite initial epidote-water fractionations that were far from equil ibrium in some experiments, diffusional exchange rates were sufficiently hi gh in this system to bracket the equilibrium fractionation within analytica l error to temperatures as low as 300 degrees C. The measured fractionation s vary systematically with temperature, decreasing from - 36 parts per thou sand at 300 degrees C to - 54 parts per thousand at 600 degrees C. Within t he experimental temperature range, these fractionations can be represented by the equation 1000 In alpha (epidote-water) = 9.3 X 10(6)/T-2 - 61.9. The present method should be applicable to other mineral-water systems and hel p to resolve some of the existing discrepancies with regard to the magnitud e and temperature dependence of hydrogen isotope fractionation factors. Cop yright (C) 1999 Elsevier Science Ltd.