Pb–Pb chronometry and the early Solar System

Of the long-lived chronometric systems, only the dual decay of 238U and 235U to 206Pb and 207Pb, respectively, have appropriate half-lives to resolve the ages of meteorites and their components formed in the first 5Myr of the Solar System. This paper reviews the theory and methods behind this chronometer, offers criteria to critically evaluate Pb–Pb ages and presents a summary of the current state and immediate future of the chronometry of the early Solar System. We recognize that there is some debate over the age of the Solar System, but conclude that an age of 4567.30±0.16Ma based on four CAIs dated individually by the same method in two different laboratories is presently the best constrained published value. We further conclude that nebular chondrules dated by the Pb–Pb method require that they formed contemporaneously with CAIs and continued to form for at least ∼4Myr, a conclusion that implies heterogeneous distribution of the short-lived 26Al nuclide in the protoplanetary disk. Planetesimals were already forming by ∼1Myr after CAI formation, consistent with their growth predominantly through the accretion of chondrules. Nebular chondrule formation was completed by ∼5Myr after CAI formation when the impact-generated Cba chondrules formed after the disk was cleared of gas and dust. We note that the absolute age of the Solar System or any single early Solar System object is not fundamental to any significant scientific question and that it is important only to know the correct relative ages of objects being used to piece together the formation history of the Solar System. As such, we point out the risks inherent in comparing Pb–Pb ages produced by different approaches in different laboratories at the level of the internal errors of individual ages. Until a cross-calibration exercise using synthetic and natural standards establishes the reproducibility between laboratories, only ages from a single laboratory, or between laboratories having demonstrated concordance, can provide a reliable relative chronometric framework for the formation and evolution of the early Solar System.