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
Abstract
A new approach to zircon dating is described that potentiallyoffers a considerable improvement in the accuracy of 207Pb/206Pbages on a wider variety of samples and at less cost than thatgenerally available from conventional techniques. Zircon isfirst preheated in a vacuum to evaporate Pb from altered domains,leaving predominantly Pb with isotopes that preserve the primaryage of the sample. Refractory Pb is removed from the zirconby thermal extraction at higher temperature directly into asilica-melt ionization activator within a thermal ionizationmass spectrometer. This produces strong ion beams that allow207Pb/206Pb isotope ratios to be measured to high precision(±10-4) with almost negligible contamination fromcommon Pb. Isotope fractionation appears to be more reproduciblethan with conventional solution analysis, allowing routine agedeterminations with precision of ~±0.2 m.y. for the Precambrian.As a test application, zircon from a noritic boundary phaseof the Sudbury impact melt gives 1849.53 ± 0.21 Ma, whilea phase from several hundred meters higher in the noritic layeris resolvably younger at 1849.11 ± 0.19 Ma (95% confidenceerrors). The enhanced precision and ease of application of thismethod should greatly increase the scientific power and availabilityof zircon dating.
- Publication:
-
Geology
- Pub Date:
- May 2008
- DOI:
- 10.1130/G24502A.1
- Bibcode:
- 2008Geo....36..383D