U/Th dating of impure carbonates: 230Th/232Th activity ratios in detrital material

Abstract

In environmental age dating the 230 Th/ 232 Th activity ratio is widely used as an indicator of detrital contamination. A clear relationship between this activity ratio and the carbonate sample's age has previously been demonstrated, whereby a detrital correction must be applied when the 230 Th/ 232 Th activity ratio of the leach drops to 20. We demonstrate that in some cases carbonate samples with 230 Th/ 232 Th well above 20 must be corrected, otherwise an overestimation of the sample's age is obtained. Evaluation of the relationship between 230 Th/ 234 U and 230 Th/ 232 Th in the carbonate's aliquots will enable evalua-tion of the limit at which a carbonate can be considered pure or impure.

Rethinking Orgnac 3 in Time and Space: Implications for Reconstructing Neandertal Origins

“…It should be noted that the 230 Th/ 232 Th activity ratios reported for these samples are all less than 20, which is considered indicating contamination by detrital materials (Michel et al 2013, Martínez-Aguirre et al 2019. Shen concluded that given the poor state of preservation of the samples, and that in 1985 this antiquity was at the upper limit of the U/Th technique, it was difÏcult to distinguish between the ages of the different samples and to rule out the possibility that the real age of that level was older than 350 ka (Shen 1985: 135-137).…”