Some footnotes to the optimization-based calibration of the ⁴⁰ Ar/ ³⁹ Ar system

Abstract A recent calibration of the 40 Ar/ 39 Ar geochronometer is based on an optimization analysis of 40 K activity data, isotopic data for the Fish Canyon sanidine (FCs) standard, and pairs of 40 Ar/ 39 Ar+ 238 U– 206 Pb data from selected samples meeting well-documented quality criteria. Inclusion of 238 U– 206 Pb data in the calibration incorporates the precisely known 238 U decay constant. Thus, this calibration is inherently consistent with the U–Pb chronometer. Initial presentation of the calibration included an inappropriate datum and should be eschewed in preference to a revision. Compared with previous calibrations, including those focusing mainly on the age of a standard (e.g. astronomical calibrations), the optimization calibration provides superior accuracy in the sense of propagated age uncertainty, particularly for ages much older than the FCs. Recent literature reveals that the optimization-based calibration has been misused and misrepresented in some cases; discussion of these cases clarifies the correct usage of the approach. Apparent conflict between 40 Ar/ 39 Ar and 238 U– 206 Pb ages for a Quaternary tuff do not appear to be a result of error in one of the three parameters determined by the optimization approach for the 40 Ar/ 39 Ar system. The optimization approach easily accommodates new constraints, but rigorous quality control is needed to maintain accuracy.