ON THE TREATMENT OF CONCORDANT URANIUM-LEAD AGES

Uranium-lead analyses that yield arguably concordant U/Pb and Pb/Pb ag es are highly reliable and often very precise; however, there is no ge nerally accepted way for either assigning a best age and age-error or for evaluating assertions of concordance. Two general methods for thes e purposes, one for samples requiring only small common Pb corrections , the other for samples requiring large common Pb corrections, are giv en in this paper. Both the Concordia age (for radiogenic-Pb/U ratios) and the Total-Pb/U isochron (a three-dimensional isochron than include s Pb-204) obviate the often dubious practice of selecting a best paren t-daughter ratio to use for a U/Pb or Pb/Pb age or isochron, make opti mum use of both decay schemes, and accommodate errors in the U decay c onstants. In addition, the Total Pb/U isochron recovers the complete s et of uranogenic Pb isotope ratios, errors, and error correlations in a single regression. Copyright (C) 1998 Elsevier Science Ltd.