Abstract
Precise measurements of the 210Pb/ 206Pb ratio in zircons permits, for the first time, the determination of complete U-Pb ages by measurement of Pb isotopes only, without determination of the U-Pb ratio by isotope dilution. The 238U daughter isotope 210Pb is generally in radioactive equilibrium with 238U; however, the very short half-life of 210Pb (22.3a) results in extremely low 210Pb/ 206Pb ratios of ∼ 10 −8, especially for Archaean zircons. These ratios have been determined using a Finnigan MAT 262® mass spectrometer equipped with a high abundance sensitivity lens system (RPQ). The accuracy of the 210Pb method has been tested on nine different zircon samples with 207Pb/ 206Pb ages from 330 to 3365 Ma corresponding to 210Pb/ 206Pb ratios between 1.31·10 −7 and 9.09·10 −9. In general, the U-Pb ages calculated using the 210Pb/ 206Pb ratio are in good agreement with conventional U-Pb age determinations on aliquots of the same sample via isotope dilution.
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