A CRETACEOUS TO PLEISTOCENE PERSPECTIVE ON INTERCALIBRATING 40Ar/39Ar AND U-PB RADIOISOTOPIC CLOCKS

To refine the Cretaceous time scale we have obtained new 40Ar/39Ar laser fusion data from sanidine in 28 ash beds deposited in the Western Interior Basin of the US and in Europe between 114 and 66 Ma. A key issue in developing this next-generation time scale is the intercalibration of the K-Ar and U-Pb radioisotopic systems. Accordingly, we have used the CA-IDTIMS method to determine the U-Pb ages of zircon in six of these Cretaceous ash beds that were deposited between 94 and 84 Ma (with measurements underway on several additional ash beds spanning the 114-66 Ma interval). The established biostratigraphy and astrochronological age models for portions of the Cretaceous period during which these ashes were deposited provide independent tests that have helped to reveal, and to avoid, some of the complications posed by inheritance or magma residence that is recorded by zircon crystals in these ash beds and assess assumptions made in the use of high-n populations to derive weighted mean 40Ar/39Ar dates. Moreover, we have obtained new 40Ar/39Ar data from sanidine in the Bishop Tuff, Huckleberry Ridge tuff, and Mesadit 4 ash bed in Morocco for comparison to published U-Pb or astronomical age determinations and/or polarity reversal timescales. Importantly, the effective elimination of Pb-loss in zircons by chemical abrasion, combined with the accurate calibration of the U-Pb system, means that U-Pb zircon and 40Ar/39Ar pairs can be used to ‘map-out’ permissible range of values for standards and constants in the 40Ar/39Ar system. Overall, these new data support the astronomically-calibrated age of 28.2 Ma for the Fish Canyon sanidine standard (Kuiper et al., 2008).