Sub-million-year age resolution of Precambrian igneous events by thermal extraction-thermal ionization mass spectrometer Pb dating of zircon; application to crystallization of the Sudbury impact melt sheet

A new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of (super 207) Pb/ (super 206) Pb ages on a wider variety of samples and at less cost than that generally available from conventional techniques. Zircon is first preheated in a vacuum to evaporate Pb from altered domains, leaving predominantly Pb with isotopes that preserve the primary age of the sample. Refractory Pb is removed from the zircon by thermal extraction at higher temperature directly into a silica-melt ionization activator within a thermal ionization mass spectrometer. This produces strong ion beams that allow (super 207) Pb/ (super 206) Pb isotope ratios to be measured to high precision (+ or -10 (super -4) ) with almost negligible contamination from common Pb. Isotope fractionation appears to be more reproducible than with conventional solution analysis, allowing routine age determinations with precision of approximately + or -0.2 m.y. for the Precambrian. As a test application, zircon from a noritic boundary phase of the Sudbury impact melt gives 1849.53 + or - 0.21 Ma, while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849.11 + or - 0.19 Ma (95% confidence errors). The enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating.