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Intercalibration between the astronomical and radio-isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated, multi-chronometer geologic timescale. Here we report a high-precision 40Ar/39Ar age for the Fish Canyon sanidine (FCs) neutron fluence monitor, by multi-collector noble gas mass spectrometry, through cross-calibration with A1 tephra sanidines (A1Ts) of the direct astronomically tuned Faneromeni section (Crete). The astronomically intercalibrated 40Ar/39Ar age of FCs of 28.172 ± 0.028 Ma (2σ, external errors) is within the uncertainty of, but more precise (± 0.10%) than, the previous 40Ar/39Ar age determined by intercalibration with astronomically tuned tephras from the Melilla Basin (Morocco). Using this proposed age for FCs, combined with measurements using the A1Ts as the neutron fluence monitor, a weighted mean Bishop Tuff 40Ar/39Ar sanidine age of 0.7674 ± 0.0022 Ma (2σ, external errors) is indistinguishable from the ID-TIMS U/Pb zircon age (0.7671 ± 0.0019 Ma). The consistency between the astronomically calibrated 40Ar/39Ar sanidine age and U/Pb zircon age for this Quaternary unit suggests that agreement between these two radio-isotopic dating techniques is now achievable at better than ± 0.3% (2σ) in the youngest part of geologic time (< 1 Ma).
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