Geochronologic significance of lead lines from old cratons

Lead isotope data from geologically mixed suites of Archaean gneisses often yield secondary isochrons that have statistically better fits than might be expected from the geological constraints on the range of samples used. Accepted as true isochrons, these Pb-Pb lines are commonly used both to determine the age of the rock suite and as evidence that all the rocks that yield data points within error of the isochron were formed at the same time from the same source. Using within geologically reasonable limits random data for initial lead and secondary lead growth, we demonstrate that heterogeneous sample sets may yield statistically valid secondary isochrons. This implies that the fit to a common secondary isochron does not necessarily mean that the samples used have the same age or an isotopically similar source. Numerical modelling of artificial data sets suggests that high correlation between mu(1) and mu(2) values is the single most important factor that enhances the statistical fit of the samples about a lead line and smoothes the initial scatter. In contrast, initial scatter will be preserved in data sets with a random relation between mu(1) and mu(2) values. The mu(1) value estimated from the lead line will be biased towards the sample with the lowest mu(2) value.