Re-evaluation of the astronomically calibrated FC sanidine age

The Fish Canyon tuff sanidine (FCs) is the most widely used standard in 40Ar/39Ar geo­chronology. Its age is determined either through intercalibration with astronomical or U/Pb dating, as intrinsic uncertainties in the K-Ar dating method are too large to provide a “primary”  FCs age. Hence, this age is not only critical for 40Ar/39Ar dating, but its determination also guarantees the intercalibration of the 3 main dating methods used to construct our standard geological time scale so that they produce the same age for the same event.In 2008, Kuiper and others published an astronomically calibrated age of 28.201 ± 0.046 Ma that apparently settled the debate as demonstrated by its subsequent incorporation in GTS2012. Yet, this age has been challenged by later studies that used various approaches, leading again to a ~1.5% age scattering. This ongoing uncertainty hampers the construction of a uniform and coherent time scale that is key to modern high-resolution, multi-disciplinary studies in Earth history.Here we combine 1) a re-examination of the astronomical tuning on which the FCs age of 28.201 Ma is based, 2) a statistical analysis of this tuning using a quantitative record and 3) new single crystal U/Pb zircon ages of the FCT and K/Pg boundary with 4) an in-depth literature review. The re-examination and statistical analysis show that the original tuning is correct and that alternative tunings consistent with different FCs ages are less plausible. Our new U/Pb ages are also in agreement with this FCs age. Finally, this outcome is consistent with studies that use single crystal 40Ar/39Ar sanidine and/or U/Pb zircon dating of usually astro­nomically dated volcanic beds and magnetic reversals of Cretaceous to Quaternary age.Alternative astronomical calibrations that are substantially younger may have suffered from less reliable radio-isotopic ages and/or uncertainties in the astronomical solution and the tuning to this solution. Recent progress in astronomical dating of the K/Pg boundary further suggest a ~100-kyr younger age, but boundary ages based on the 3 main dating methods remain inconsistent. An older FCs age based on statistical optimization may result from the inclusion of 40Ar/39Ar-U/Pb data pairs that do not follow Earthtime protocols, but is in itself an elegant approach to tackle both decay constants and standard issues.In summary the fundamental issue of the FCs age has still not been solved but consensus is being reached on an age close to 28.2 Ma. We therefore recommend the continued use of the astronomically calibrated age of 28.201 ± 0.046 Ma; this recommendation is in line with the recent decision to adopt this age again in GTS2020. However, we endorse further investigation in a community-based effort, where new data and improved methodologies may lead to better insight into fundamental properties and a slightly different age of the standard. This effort will also reveal whether lternative approaches, such as statistical optimization or direct U/Pb FCT zircon dating, may provide an even more accurate and precise FCs age.