A lead isotope method for the accurate dating of disturbed geologic systems: Numerical demonstrations, some applications and implications

Lead isotopes are compared in two equivalent diagrams. With a range in U/Pb, a closed system that evolved its radiogenic Pb in a single stage yields data that define a line of exactly the same age in the two presentations. This strict reproducibility (within +/- a few 10(6) yr, at most) is the crux of the concept of Pb-isotope synchronism. In contrast, an open system produces data that disperse variably in the two diagrams, yielding ages which are often different by much greater than 107 yr. However, because an age-calculation reconciles uncertainty in the age with the degree of dispersion in the data, a highly disturbed system yields false ages (from both diagrams), which nevertheless overlap within the calculated errors.