THE SIGNIFICANCE OF HYDROGEN AND OXYGEN STABLE ISOTOPIC FRACTIONATIONS BETWEEN CARNALLITE AND BRINE AT LOW TEMPERATURE - EXPERIMENTAL AND EMPIRICAL RESULTS

摘要

Fractionation of hydrogen and oxygen isotopes between the hydration water of carnallite (KMgCl3 . 6H(2)O) and brines are evaluated by exchange and synthesis of carnallite in the laboratory at temperatures between 22 and 45 degrees C, and by analyses of carnallites from natural environments. Hydration water in carnallite is depleted in D relative to brine from which carnallite precipitates. The temperature dependence of this fractionation can be expressed by 1000 ln alpha(carnallite-H2O) = -18.4(10(6)T(-2)) + 162. In contrast to the fractionation of hydrogen isotopes, oxygen isotopes do not fractionate between carnallite and brine over this range in temperature. Exchange experiments indicate that the rate of exchange of hydrogen and oxygen isotopes between carnallite and brine is significant, with t(1/2) approximate to 2 y. Empirical fractionations calculated from the stable isotopic compositions of recrystallized carnallites in a natural evaporite deposit agree with those calculated by mineral synthesis. These results indicate that the stable isotopic composition of hydration water of carnallite can be used to determine the isotopic compositions, temperatures, and sources of fluid from which they originally formed or recrystallized. [References: 24]