Revisiting the age of the Fish Canyon sanidine dating standard

In 2008, Kuiper and others published an astronomically calibrated age of 28.201 ± 0.046 Ma for the Fish Canyon sanidine (FCs), the most widely used standard in Ar/Ar dating. This age was incorporated in GTS2012 and GTS2020, but has been challenged by later studies that used various approaches, leading again to a ∼ 1.5 % age scattering. This uncertainty hampers the construction of a uniform and coherent time scale that is key to modern high-resolution, multi-disciplinary studies in Earth history. To solve this ongoing uncertainty, we present 1) a visual and statistical re-examination of the astronomical tuning on which the FCs age of 28.201 Ma is based and 2) new single crystal U/Pb ID-TIMS zircon ages of the astronomically calibrated Faneromeni A1 ash bed and the Fish Canyon Tuff (FCT). Our results corroborate the initial tuning and invalidate astronomical and U/Pb based FCs age calibrations, which are much younger or older. The preferred astronomical calibration seems to converge to a slightly younger age of 28.171–28.176 Ma. This is in good agreement with our new Bayesian zircon eruption age of 28.171 + 0.039/− 0.044 Ma for the FCT and the recently published Bayesian calibration of the 40 K decay scheme with a coupled FCs age of 28.183 ± 0.070 Ma. Importantly, the astronomical and U/Pb-based calibrations now yield similar ages, implying mutual agreement between the three main dating methods to construct our time scale. However, by contrast, the Cretaceous/Paleogene (K/Pg) boundary, which represents a critical triple point for the intercalibration, now produces divergent ages. In summary, much progress has been made in solving the critical issue of the age of the FCs dating standard. For the moment, we either recommend the continued use of the astronomically calibrated age of 28.201 ± 0.046 Ma or, preferentially, a slightly younger age of 28.171–28.176 Ma. We further endorse investigation in a community-based effort where new data and improved methodologies may lead to further insight into fundamental properties and potentially a definitive age not only for the FCs dating standard, but importantly also for the K/Pg boundary.