A new approach to Pb evaporation and condensation from zircon enabling sub-Ma age determination on Precambrian rocks

• A new approach is presented for Pb–Pb dating of Precambrian zircon.
• Rapid evaporative extraction of undisturbed Pb
• The ability to double spike for high age precision
• Does not require laborious cleaning or an extensive clean air environment
• Factor of 2–3 improvement in age precision on Precambrian rocks

Sub-million year age precision on low to moderately metamict Precambrian zircon is often reproducible using the isotope dilution thermal ionization technique (ID-TIMS) exploiting U–Pb double spiking for precise mass fractionation correction. However, this requires total dissolution of sample and laborious chemical processing. On the other hand whole grain evaporation techniques of Pb extraction are simple and less time consuming but provide relatively inefficient ionization and are not amenable to double Pb isotope spiking for precise mass fractionation correction, which limits the precision of ages that they can provide (often restricted to ±few million years). A modified evaporation–condensation method is described that overcomes these limitations with the ability to use 202Pb–205Pb double spiking for precise mass fractionation correction in order to achieve sub-million year age precision on low to highly metamict Precambrian samples. Examples are shown of age determinations on previously dated zircon populations from the Marmion tonalite and Nevison Lake rhyolite, western Superior province (U < 200 ppm), which give 3001.8 ± 0.5 Ma and 2998.2 ± 0.3 Ma, respectively, and from the black norite and the felsic norite in the Sudbury igneous complex (U > 1000 ppm), which give 1848.9 ± 0.2 Ma and 1851.3 ± 0.5 Ma, respectively (all errors are 95% confidence).