TIME-SCALE CALIBRATION BY HIGH-PRECISION U-PB ZIRCON DATING OF INTERSTRATIFIED VOLCANIC ASHES IN THE ORDOVICIAN AND LOWER SILURIAN STRATOTYPES OF BRITAIN

High initial parent/daughter element ratios and a unique dual decay scheme make UPb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of UPb dating, we have analyzed microgram-size (2-12 × 10-8 g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2 Ma). Four of the stratotype samples from Britain for which fission-track ages [1] were previously reported have yielded improved ages of: 438.7 ± 2.0 Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland; 457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and; 465.7 ± 2.1 and 464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps (P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at 445.7 ± 2.4 Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is 453.7 ± 1.8 Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain. © 1990.