QUATERNARY GEOCHRONOLOGY USING THE U-TH-PB METHOD

We describe a method of uranium-thorium-lead (U-Th-Pb) isotopic age da ting for Quaternary rocks. The approach uses an instrumental mass disc rimination correction for lead isotope ratios, which allows small enri chments of radiogenic Pb-206 and Pb-208 to be detected at the level of 0.001%. Igneous rocks hosting minerals with a range in U-238/Pb-204 v alues of 100 can be dated with uncertainties of approximately +/- 15-2 0 kyr. A Quatemary rhyolite dated at 1.19 Ma by K-Ar yields a U-238-Pb -206 age of 1.03 +/- 0.10 Ma. A Holocene dacite (9.5 ka) has uniform P b-206/Pb-207 to within +/- 0.0015% in groundmass phases, but 1 mm plag ioclase phenocrysts have lower Pb-206/Pb-207 by 0.105 +/- 0.002% indic ating contamination of the magma after plagioclase crystallization. Hi gh precision Pb-206/Pb-207 ratios may be a useful new tool for petroge netic studies.