An evaluation of several methods for determining [formula omitted] ages in impure carbonates

Abstract

Four different schemes have been used to correct the U-series ratios found in the leachates of impure carbonates so that an age may be calculated from them. The present work compares the performance of three of these, Schemes I, II and SL, with that of the fourth, Scheme L/L. Scheme L/L had been shown previously to yield valid ages in a comprehensive study of thirty sample sets comprising 170 samples of impure carbonates from the Dead Sea basin. This scheme performs the isotopic correction by comparing the ratios found in the leachates of several coeval samples. The validity of two of the other schemes, Scheme I and II, was tested by comparing their results with the Scheme L/L results on eight of the same thirty sample sets. Schemes I and II perform corrections by comparing the isotope ratios in the leachate and residue of a single impure carbonate; the two schemes differ in their assumptions regarding the leachate-residue relationship and, hence, in their mathematical treatments of the isotopic ratios. Comparison of Scheme L/L with I and II shows that the latter are invalid in more than half the cases. The fourth correction scheme, Scheme SL, performs the corrections on the isotopic ratios of a single leachate by assuming that the correction factor, R 0 (the original detrital 230Th 232Th ratio) has a more or less constant worldwide value, usually taken to be 1.5–1.7. The present study shows that R 0 is more like 3.3 for the Dead Sea basin and evaluates the errors incurred if the incorrect R 0 is used. Finally, an attempt is made to arrive at the best worldwide estimate of R 0 and its uncertainty for any new unstudied area and a graph is presented by means of which one can estimate the consequent age uncertainty.