Isotopic fractionation during leaching of impure carbonates and their effect on uranium series dating

Description

Experiments were designed to evaluate the behaviour of detritus during the leaching of impure carbonates by applying the U-Th isochron technique. We used one natural detritus and a pure, well-dated CaCO3 , alone and in artificial mixtures. One set of experiments was designed to study the effects of sample pre-treatment on the detritus and a second set to examine the effect of acid attacks on both the carbonate and the detritus fractions in a mixture of constant proportions. For both sets of data, the extracted fraction from the detritus was calculated for 238U, 234U, 230Th and 232Th and their constancy established. These conditions are necessary to apply the L/L method [Schwarcz, H.P., Latham, A.G., 1989. Dirty calcites. 1.–Uranium-series dating of contaminated calcite using leachates alone. Chemical Geology (Isotopes Geoscience Section) 80, 35–43] for U/Th dating. Finally, the L/L method was applied to the artificial impure carbonate and the resulting age is found in agreement with the age of the pure carbonate sample. We concluded that conditions of the L/L method to be applied for ageing impure carbonates hold.

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