DETERMINATION OF U-PB AGES OF ZIRCONS BY DIRECT MEASUREMENT OF THE PB-210 PB-206 RATIO/

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