Revised astronomically calibrated 40Ar/39Ar ages for the Fish Canyon Tuff sanidine - Closing the interlaboratory gap

The 40Ar/39Ar geochronology method is capable of high precision (<0.05%), but remains limited by relatively large uncertainties in K-40 decay constants and the ages of natural reference mineral standards. The most widely used 40Ar/39Ar reference mineral is the well-known ca. 28 Ma Fish Canyon Tuff sanidine (FCTs). Several studies have attempted to calibrate FCTs against astronomically tuned tephra in Crete (Faneromeni A1 tephra) and Morocco (Messadit Mes4 tuff) as well as deep-sea sedimentary sequences. Previously reported astronomically tuned ages range from 28.126 +/- 0.019 to 28.21 +/- 0.18 Ma (2 sigma), a range of similar to 0.3%, compared to precision levels of <0.05% achievable by new generation, multi-collector mass spectrometer systems. In this study, we revisit the astronomical calibration of FCTs. Relative to ages of 6.943 +/- 0.005 Ma for A1 tuff sanidine (A1Ts) and 6.791 +/- 0.010 Ma (2 sigma) for Mes4 tuff sanidine (Mes4Ts), we calculate revised astronomically tuned ages for Fish Canyon Tuff sanidine of 28.175 +/- 0.012 Ma (+/- 0.023 Ma, including the uncertainty in the age of A1Ts) and 28.176 +/- 0.010 Ma (+/- 0.042 Ma, including the uncertainty in the age of Mes4Ts), respectively (assuming negligible differential 39Ar recoil loss). This age is within uncertainty of most recent astronomical intercalibrations, permitting calculation of inter-laboratory mean ages of 28.176 +/- 0.010 (+/- 0.023) Ma and 28.179 +/- 0.009 (+0.042) Ma, respectively. As the astronomical age of the A1 tuff is more precise than that of the Mes4 tuff, we recommend that the former value is adopted as the astronomical age for FCTs. This age is consistent with available U-Pb zircon age data, but remains distinctly older than recent astronomical ages of 28.10 and 28.150 Ma inferred from deepsea Ocean Drilling Program sediments, indicating that further work is required to align the astronomical tuning of terrestrial versus deep-sea sediments. Based on previous R-values for the Alder Creek Rhyolite sanidine (ACRs) and Mount Dromedary biotite (MD-2b) reference minerals, co-irradiated with FCTs, we calculate revised astronomically calibrated ages of 1.18342 +/- 0.00069 (+/- 0.009) Ma for ACRs and 99.323 +/- 0.077 (+/- 0.33) Ma for MD-2b, the latter amended to 99.20 +/- 0.01 (+/- 0.38) Ma to account for relative recoil loss of 39Ar K. To further enhance the accuracy of 40Ar/39Ar ages, our study also highlights the need to carefully control neutron fluence gradients and consider recoil effects.