Measurements of the uranium concentration and distribution in a fossil equid tooth using fission tracks, TIMS and laser ablation ICPMS: Implications for ESR dating

Abstract

Combined ESR/U-series dating of tooth enamel is important for obtaining chronological constraints on archaeological sites. The precision and accuracy of this method depend on the uncertainties associated with the equivalent dose and the dose rate. The precision of the dose rate calculation, in turn, depends on a number of parameters such as the internal radionuclide concentrations and distributions in the enamel, dentine and cementum and in the surrounding sediments as well as on the cosmogenic dose rate. Furthermore, teeth are open systems for U and therefore constraints on the U-uptake model are needed. The combination of all the uncertainties results in uncertainties of ∼ 10 % on the age at 95% confidence level. This can, however, be improved by reducing the uncertainties on the internal radionuclide concentrations and distributions. We present a comparison of three methods for measuring the U concentrations in dental tissue of a 40 ka old equid tooth. Fission tracks are used to map the U distribution with high spatial resolution. The U and Th concentrations and isotopic compositions were measured with high precision but poor spatial resolution using TIMS. It is demonstrated that in situ LA-ICPMS provides an effective method for precise measurements of the U concentration at high spatial resolution. This method also has high potential for isotopic U-series measurements using MC-ICPMS. The results of the U measurements are used to constrain the effective alpha- and beta-dose rates in enamel sampled for ESR analysis. The TIMS U–Th isotope concentrations and in particular the 230Th concentrations are used to constrain the U uptake for combined ESR/U-series dating.