New SHRIMP zircon ages from tuffs within the British Palaeozoic stratotypes

Two different approaches to deconvolving mixed age populations resolve SIMS ages within the TEMORA reference zircon into a small slightly older group and a large group that is well fitted within the calculated age uncertainties. We discard the older group as spurious rather than increase the uncertainties to try to obtain a (statistically) single propulation. This approach minimizes errors in the sample component ages and allows more reliable detection of potentially real age differences. The R33 reference zircon was run to give an explicit measure of accuracy relative to TEMORA but results are indecisive owing to probable Pb loss. Zircons from four tuff horizons within the lower Palaeozoic British stratotypes were re-dated using SIMS to explore the possible presence of slightly older detrital zircons. Two tuff horizons, the Llyfnant Flags (Floian) and Serw Formation (Darriwilian), have single age components, and the Pont-y-Ceunant Ash (Katian) and Birkhill Shale (lower Llandovery) have two components interpreted to be volcanic and slightly older inherited zircons. These and previous SHRIMP ages referenced to the SL13 standard confirm that isotope dilution ages for the same tuffs need revision.