Uranium-Series Dating of Carbonates from Bir Tarfawi and Bir Sahara East

Abstract

Uranium-series dating is based on the fact that the radioactive decay of uranium leads to the formation of two chains of radioactive isotopes whose half-lives are comparable to the time-scale of the Pleistocene. If a U-bearing sample is left undisturbed sufficiently long, then each of these isotopes grows into “secular equilibrium” with the parent U isotopes, ²³⁸U and ²³⁵U. That is, the activity (measured in disintegrations per minute per gram) of each isotope becomes numerically equal to that of the parent. The time needed to reach this state is commensurate with the half-life of the progeny isotope. It has been observed that uranium is readily separated from its progeny isotopes, for example, during the precipitation of a carbonate mineral. Therefore, we can determine the age of such a mineral by measuring the ratio of the progeny to the parent, since the progeny isotope will have been generated in situ by radioactive decay. For example, the thorium isotope ²³⁰Th (t_1/2 = 75.2 kyr), progeny of ²³⁸U, is usually not present in freshly deposited calcite, and the age of a fossil calcite sample can be determined from the ²³⁰Th²³⁸U ratio. In general, ²³⁴U is not in equilibrium with its parent, ²³⁸U, and the age of a sample is better determined from the ratio ²³⁰Th/²³⁴U.