Joint determination of 40K decay constants and 40Ar∗/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology

40Ar/39Ar and K–Ar geochronology have long suffered from large systematic errors arising from imprecise K and Ar isotopic data for standards and imprecisely determined decay constants for the branched decay of 40K by electron capture and β− emission. This study presents a statistical optimization approach allowing constraints from 40K activity data, K–Ar isotopic data, and pairs of 238U–206Pb and 40Ar/39Ar data for rigorously selected rocks to be used as inputs for estimating the partial decay constants (λε and λβ) of 40K and the 40Ar∗/40K ratio (κFCs) of the widely used Fish Canyon sanidine (FCs) standard. This yields values of κFCs = (1.6418 ± 0.0045) × 10−3, λε = (0.5755 ± 0.0016) × 10−10 a−1 and λβ = (4.9737 ± 0.0093) × 10−10 a−1. These results improve uncertainties in the decay constants by a factor of >4 relative to values derived from activity data alone. Uncertainties in these variables determined by our approach are moderately to highly correlated (cov(κFCs, λε) = 7.1889 × 10−19, cov(κFCs, λβ) = −7.1390 × 10−19, cov(λε, λβ) = −3.4497 × 10−26) and one must take account of the covariances in error propagation by either linear or Monte Carlo methods. 40Ar/39Ar age errors estimated from these results are significantly reduced relative to previous calibrations. Also, age errors are smaller for a comparable level of isotopic measurement precision than those produced by the 238U/206Pb system, because the 40Ar/39Ar system is now jointly calibrated by both the 40K and 238U decay constants, and because λε(40K) < λ(238U). Based on this new calibration, the age of the widely used Fish Canyon sanidine standard is 28.305 ± 0.036 Ma. The increased accuracy of 40Ar/39Ar ages is now adequate to provide meaningful validation of high-precision U/Pb or astronomical tuning ages in cases where closed system behavior of K and Ar can be established.