TIME SCALE CALIBRATION BENEFITTING FROM THE MATTINSON TECHNIQUE (CA-TIMS) APPLIED TO VOLCANIC ZIRCONS

Radioisotopic ages play a central role in calibrating events and processes in Earth’s history and U-Pb zircon geochronology is regarded as one of the most prominent geochronological techniques. Analytical advances have led towards the capability of precisely analyzing minute amounts of samples; however, eliminating the age reducing effects of Pb loss have been notoriously difficult which essentially made the use of ²⁰⁶Pb/²³⁸U ages (and thus their application for Phanerozoic geochronology) unacceptably fragile. Jim Mattinson’s innovative research using a combination of annealing and leaching techniques (CA-TIMS) in order to isolate portions of zircons that are devoid of Pb loss has recently led to a paradigm shift in U-Pb zircon geochronology such that we can now extract accurate ²⁰⁶Pb/²³⁸U ages from radiation damaged zircons.

Recent examples from Permian mass extinctions strikingly illustrate how Jim Mattinson’s research has led to addressing and solving first order geological question of cause and effect and the synchroneity of events. The study of Triassic evolutionary and environmental events (e.g. early dinosaur evolution) has been greatly hampered by the lack of radioisotopic ages in both terrestrial and marine strata, making correlations between these environments impossible. We present new U-Pb ages from Triassic tuffs within marine strata in New Zealand. Initial zircon ages from a tuff within upper Etalian strata yield an age of 237 Ma, demonstrating that the Etalian-Kaihikuan boundary may not be correlative to the Ansian-Ladinian boundary (>240 Ma) as previously assumed. Zircons from a tuff bed intercalated with Otamitan (Norian) shell beds rich in Manticula problematica yield a coherent cluster of zircon ages with a weighted mean of 218 Ma (permil level uncertainties), which is the first robust age constraint on marine Norian sediments. The latter age, in combination with new U-Pb zircon ages from the terrestrial Chinle Fm., is instrumental in generating a chronostratgraphic framework that tests ambiguous correlations between different parts of the world. Our new ages are in contrast to previous results but more coherent. They demonstrate the significance of the CA-TIMS technique that now enables us to obtain accurate and unambiguous U-Pb ages from complex zircon populations.