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

• Published in: Geochimica et cosmochimica acta 2010-09, Vol.74 (18), p.5349-5367
• Main Authors: Renne, Paul R., Mundil, Roland, Balco, Greg, Min, Kyoungwon, Ludwig, Kenneth R.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.06.017

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 ( λ ε and λ β ) of 40K and the 40Ar∗/ 40K ratio ( κ FCs) of the widely used Fish Canyon sanidine (FCs) standard. This yields values of κ FCs = (1.6418 ± 0.0045) × 10 −3, λ ε = (0.5755 ± 0.0016) × 10 −10 a −1 and λ β = (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( κ FCs, λ ε ) = 7.1889 × 10 −19, cov( κ FCs, λ β ) = −7.1390 × 10 −19, cov( λ ε , λ β ) = −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 λ ε ( 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.