SIMULTANEOUS DETERMINATION OF 40K DECAY CONSTANTS AND AGE OF THE FISH CANYON SANIDINE 40AR/39AR STANDARD

Uncertainties in the decay constants for electron capture (λ_ε) and beta (λ_β) decay of ⁴⁰K, and in the ages of standards, have limited the accuracy of ⁴⁰Ar/³⁹Ar geochronology to no better than 1% of the age (1σ here and throughout). The exemplary possible precision and unsurpassed time range of applicability of the technique are thus to some extent compromised for dating absolute time, but these very qualities provide incentive to improve the situation. Recent astronomical calibration of the age of the Fish Canyon sanidine (FCs) standard by Kuiper et al. (2008) partially resolves the problem but does not address uncertainties in the decay constants. Here we present a refinement of the approach of Kwon et al. (2002), which uses ⁴⁰Ar/³⁹Ar ages relative to FCs paired with independent age constraints for a number of volcanic rocks to simultaneously determine the age of FCs and the total decay constant of ⁴⁰K. We now extend the approach to three variables: 1) the electron capture constant; 2) the beta decay constant; and 3) the ⁴⁰Ar*/⁴⁰K ratio of FCs, termed K. Several types of constraints are used: 1) 14 pairs of ⁴⁰Ar/³⁹Ar and U/Pb data for samples ranging from 126 Ma to 2.07 Ga; 2) activity data for electron capture decay; 3) activity data for beta decay; 4) liquid scintillation counting (LSC) data for total ⁴⁰K activity; 5) independently calibrated K measurements for FCs. These constraints can be visualized as parametric surfaces in 3-space whose best-fit intersection simultaneously defines unique values of the two decay constants and the value of K for FCs. Currently available data yield: K = (1.6518 +/- 0.0019) x 10^-3 (0.12%); λ_ε = (5.7926 +/- 0.0066) x 10^-11 (0.11%); λ_β = (4.9647 +/- 0.0109) x 10^-10 (0.22%). These results place the ⁴⁰Ar/³⁹Ar method on comparable ground, in terms of accuracy, to the U/Pb method and indicate an age of 28.293 ±0.046 Ma for FCs, validating (at 95% conf.) the result of Kuiper et al. (2008). Better constraints on λ_β may be provided by additional ⁴⁰Ar/³⁹Ar-U/Pb data pairs for volcanic rocks >2 Ga, but the paucity of undisturbed such samples meeting stringent ⁴⁰Ar/³⁹Ar data quality standards hinders this goal. Fortunately, the relative imprecision of λ_β is of minor importance for most terrestrial samples dated by⁴⁰Ar/³⁹Ar methods where time scale grade accuracy is required.