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. The decay of 40K to the stable isotopes 40Ca and 40Ar is used as a measure of time for both the K-Ca and K-Argeochronometers, the latter of which is most generally utilized by thevariant 40Ar∕39Ar system. The increasing precision ofgeochronology has forced practitioners to deal with the systematicuncertainties rooted in all radioisotope dating methods. A major componentof these systematic uncertainties for the K-Ar and 40Ar∕39Ar techniques is imprecisely determined decay constants and an incompleteknowledge of the decay scheme of 40K . Recent geochronology studiesquestion whether 40K can decay to 40Ar via an electron capturedirectly to ground state (EC ground ), citing the lack of experimentalverification as reasoning for its omission. In this study, we (1) provide atheoretical argument in favor of the presence of this decay mode and (2) evaluate the magnitude of this decay mode by calculating the electroncapture to positron ratio ( EC ground / β + ) and comparingcalculated ratios to previously published calculations, which yield EC ground / β + between 150–212. We provide support for thiscalculation through comparison of the experimentally verifiedEC ground / β + ratio of 22Na with our calculation usingthe theory of β decay. When combined with measured values of β+ and β− decay rates, the best estimate for thecalculated EC ground / β + for 40K yields a partial decayconstant for 40K direct to ground-state 40Ar of 11.6 ± 1.5 × 10 - 13  a −1 (2 σ ). We calculate a partial decayconstant of 40K to 40Ar of 0.592 ± 0.014 × 10 - 10  a −1 and a total decay constant of 5.475 ± 0.107 × 10 - 10  a −1 (2 σ ), and we conclude that although omission ofthis decay mode can be significant for K-Ar dating, it is minor for 40Ar∕39Ar geochronology and is therefore unlikely to havesignificantly biased published measurements.

geochronology

Production of &amp;lt;sup&amp;gt;40&amp;lt;/sup&amp;gt;Ar by an overlooked mode of &amp;lt;sup&amp;gt;40&amp;lt;/sup&amp;gt;K decay with implications for K-Ar geochronology