²³⁸ U/ ²³⁵ U Systematics in Terrestrial Uranium-Bearing Minerals

A Better Date Uranium-lead (U-Pb) dating, which is one of the most commonly used methods of radiometric dating for old terrestrial materials, operates by comparing the ratio of trace levels of U with its nuclear decay product Pb. This dating method, and the similar Pb-Pb method, assumes that the ratio between the two most common U isotopes ( 238 U and 235 U) is constant. By accurately measuring the 238 U/ 235 U ratio in a suite of minerals representing a range of tectonic environments, Hiess et al. (p. 1610 ; see the Perspective by Stirling ) demonstrate that this ratio is more variable than was previously thought. The variability does not reflect any systematic bias with time, location, or temperature—suggesting that ideally 238 U/ 235 U should be determined for every sample to calculate ages. In the absence of such data, a revised 238 U/ 235 U ratio for zircon minerals could significantly modify previous age estimates using U-Pb and Pb-Pb dating techniques.