An Integrated Model for Lead Isotopic Evolution for Samples from the Canadian Shield

Abstract

Values reported during the last 15 years for lead isotopic ratios of 10 suites of samples were normalized to absolute values. Inferior, duplicating sets of analyses were discarded. It was postulated that the broad features of the isotopic patterns should be explicable in terms of a small number of major tectonic events. A table of times for these events was prepared on the basis of isochron fits to the individual suites. The times correspond closely with the original interpretations made for the suites.The 30 possible time values were reduced to six distinct events. A modified two-stage model was used to represent each suite and times were selected for the model from the six events, by choosing those events that best described the suite. Residuals were calculated corresponding to a simultaneous least squares fit of all the data. From the minimum value of the sum of the residuals, optimum time values were obtained of 3470 ± 60, 3040 ± 10, 2630 ± 20, 2250 ± 20, 1500 ± 40, 920 ± 30 m.y. Uncertainties quoted are approximately one-sigma values. Thus it was possible to fit all the Canadian Precambrian data used in this study with six time values.The parameters μ = 8.79, t0 = 4580 m.y., a0 = 9.346, b0 = 10.218 as found by Cooper et al. were used for the first stage of the model.