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 K-40 by electron capture and beta-emission. This study presents a statistical optimization approach allowing constraints from K-40 activity data, K-Ar isotopic data, and pairs of U-238-Pb-206 and a 40Ar/39Ar data for rigorously selected rocks to be used as inputs for estimating the partial decay constants (lambda(epsilon), and lambda(beta)) of K-40 and the 40Ar*/K-40 ratio (kappa(FCs)) of the widely used Fish Canyon sanidine (FCs) standard. This yields values of kappa(FCs) = (1.6418 +/- 0.0045) x 10(-3),lambda(epsilon) = (0.5755 +/- 0.0016) x 10(-10) a(-1) and lambda(beta) = (4.9737 +/- 0.0093) x 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(kappa(FCs), lambda(epsilon)) = 7.1889 x 10(-19), cov(kappa(FCs,) lambda(beta)) = -7.1390 x 10(-19) cov(lambda(epsilon), lambda(beta)) = -3.4497 x 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 U-238/Pb-206 system, because the 40Ar/39Ar system is now jointly calibrated by both the K-40 and U-238 decay constants, and because lambda(epsilon)(K-40) < lambda(U-238). 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. (C) 2010 Elsevier Ltd. All rights reserved.