Production of <mml:msup>40</mml:msup>Ar by an overlooked mode of <mml:msup>40</mml:msup>K decay withimplications for K-Ar geochronology

The decay of K-40 to the stable isotopes Ca-40 and Ar-40 is used as a measure of time for both the K-Ca and K-Ar geochronometers, the latter of which is most generally utilized by the variant Ar-40/Ar-39 system. The increasing precision of geochronology has forced practitioners to deal with the systematic uncertainties rooted in all radioisotope dating methods. A major component of these systematic uncertainties for the K-Ar and Ar-40/Ar-39 techniques is imprecisely determined decay constants and an incomplete knowledge of the decay scheme of K-40. Recent geochronology studies question whether K-40 can decay to Ar-40 via an electron capture directly to ground state (ECground), citing the lack of experimental verification as reasoning for its omission. In this study, we (1) provide a theoretical argument in favor of the presence of this decay mode and (2) evaluate the magnitude of this decay mode by calculating the electron capture to positron ratio (ECground) and comparing calculated ratios to previously published calculations, which yield between 150-212. We provide support for this calculation through comparison of the experimentally verified ECground /beta(+)ratio of Na-22 with our calculation using the theory of beta decay. When combined with measured values of beta(+) and beta(-) decay rates, the best estimate for the calculated ECground /beta(+) for K-40 yields a partial decay constant for K-40 direct to ground-state Ar-40 of 11.6 +/- 1.5x 10(-13)a(-1) (2 sigma). We calculate a partial decay constant of K-40 to Ar-40 of a(-1) and a total decay constant of a(-1) (2 sigma), and we conclude that although omission of this decay mode can be significant for K-Ar dating, it is minor for Ar-40/Ar-39 geochronology and is therefore unlikely to have significantly biased published measurements.