IMPROVING CRETACEOUS TIME SCALE UNCERTAINTIES VIA MULTI-COLLECTOR 40AR/39AR DATING

The recent integration of chemostratigraphy, biostratigraphy and astrochronology together with radioisotopic dating of Upper Cretaceous strata via the ⁴⁰Ar/³⁹Ar and U-Pb methods, resulted in a Cenomanian-Santonian time scale of unprecedented precision, accuracy, and global exportability. While the precision of ²³⁸U-²⁰⁶Pb dates from individual zircons is near the 0.1% level and allows for distinguishing older, inherited or antecryst zircons from those grown in the erupted magma, the ⁴⁰Ar/³⁹Ar dates from single sanidine crystals have uncertainties of 1-4% necessitating calculation of weighted mean ages from large-n data sets. The latter reflects the acquisition of ⁴⁰Ar/³⁹Ar data using a single-collector mass spectrometer with an analog electron multiplier and mass resolution of 550. Using a next-generation 5-collector Noblesse mass spectrometer with ion counting electron multipliers that are 50 times more sensitive, with a mass resolving power of 3000, we now obtain dates from individual Cretaceous sanidines with a precision of 0.07%. This rivals the best ID-TIMS U-Pb zircon dates, and in samples from near the Cenomanian-Turonian boundary in the Western Interior Basin, allows older inherited sanidine crystals to be readily distinguished from younger populations. Not only are the weighted mean depositional/eruptive ages an order of magnitude more precise than those from the older mass spectrometer, they were acquired in far less time on as few as a dozen crystals. The new dates indicate that many zircon and sanidine crystals precisely record time since eruption. However, the greatly improved matches between new ⁴⁰Ar/³⁹Ar and U-Pb ages now lead us to consider alternative correlations of some ash beds, discussed in Meyers et al. (2012; Geology). The precision of the new ⁴⁰Ar/³⁹Ar ages, together with ID-TIMS U-Pb ages, vividly illustrates how modern radioisotopic geochronology is driving a re-examination of stratigraphic data to reveal previously obscure hiatuses, or the uncertainty in the positions of ash beds relative to biozone boundaries. We thus aim to refine the age model for the Cenomanian-Turonian boundary interval through the unification of state-of-the-art ⁴⁰Ar/³⁹Ar and U-Pb dates with astrochronology.