SIMS U-Pb zircon ages for the Upper Devonian Snobs Creek and Cerberean Volcanics from Victoria, with age uncertainty based on UO2/UO v. UO/U precision

摘要

Sensitive high-resolution ion microprobe (SHRIMP) zircon 206Pb/238U 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 (2). 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+/UO₂+ 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 UO₂/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.