The use of cogenetic uranium-lead isotope systems in zircons in geochronology

Among the more common natural radioactivity relationships investigated in geochronology the paired uranium-lead isotope systems have always been of interest for the multiple isotopic information yielded in a single study. Even in the earliest investigations of Nier (1939) it was apparent, however, that disagreement between the apparent ages calculated from the Pb²⁰⁶/U²³⁸ and Pb²⁰⁷/U²³⁵ ratios was a common phenomenon. It has been definitely established that, in general, errors in analytical techniques or in the accepted values of the decay constants U²³⁸ and U²³⁵ of U and U are not the sources of the more serious discrepancies. It is clear that this type of age discordance is a reflection of natural disturbances of the isotopic systems during their geologic history.