Abstract
Sensitive high-resolution ion microprobe (SHRIMP) zircon 206 Pb/ 238 U ages for the Snobs Creek Volcanics and the overlying Cerberean Volcanics in central Victoria are analytically indistinguishable at 373.2 ± 1.8 and 375.0 ± 2.0 Ma (2σ). The SHRIMP data provide a refined numerical age for interbedded freshwater fish fauna of the Taggerty Formation and, together with its recently revised Givetian age based on independent faunal correlations, provide a new calibration point on the Devonian time scale. The use of Pb + /UO 2 + rather than Pb + /U + in secondary ionization mass spectrometry (SIMS) dating reduces the range of secondary ion discrimination from zircon. Between-spot correlations observed between UO/U and UO 2 /UO ratios give quantitative estimates for external SIMS uncertainty per spot age that are independent of assumed uniformity in zircon Pb/U. They can exceed internal uncertainties by up to 1.5% dependent on the analytical session. This has important effects on the calibration of SIMS ages at the sub-percent level using imperfect reference zircons subject to small-scale perturbations of the U/Pb system.
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