Dating galenas by means of their isotopic constitutions

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Abstract
A number of isotopic analyses of ore leads from Precambrian deposits have been made. These leads have been classed as “ordinary” or “anomalous” on the basis of their isotopic compositions. The authors suggest that ordinary leads may be dated by comparing their isotopic abundances with average lead abundance‐time curves provided that a possible error is assigned which is sufficiently large to include the effects of irregularities of the uranium, thorium, and lead distribution in the Earth's crust. These errors have been calculated for two Earth models which seem physically reasonable. In both cases the errors are sufficiently small for all Precambrian specimens so that the ages may be useful. For early Precambrian samples the errors are comparable to those for standard radioactive age determinations. Galenas which have been dated in this way give ages which seem reasonable in view of their geological environment.The anomalous type of ore leads can be explained by large additions of radiogenic lead to ordinary lead, and the lead abundance ratios can be used in a least‐squares calculation to provide a limit to the time of separation of the leads from the source of the radiogenic additions. This method is applied to a suite of samples from the Sudbury, Ontario, area.