Percent-Level Production of 40Ar by an Overlooked Mode of 40K Decay
Abstract
The K-Ar and K-Ca geochronometers are based on the radioactive decay of 40K, a naturally occurring radioisotope that undergoes branched decay to 40Ar and 40Ca with a total half-life of ~1.3 Ga. The K-Ar system is now most often exploited using the 40Ar/39Ar method. The 40Ar/39Ar technique has now matured to the point that some small but increasingly important systematic uncertainties need to be quantified and/or revisited, including those that arise from the measured decay rate of 40K.
There is consensus that most 40K decays via 𝞫- to 40Ca or by EC to 40Ar via an excited state, accompanied by 1.46 MeV 𝞬 emission, both of which are straightforward to measure directly. A geochronologically insignificant amount (~ 0.001%) of 40K is also known to decay to 40Ar via 𝞫+, which has been measured directly (Engelkemeir 1962, Phys. Rev. v126 p1818). However, there is still uncertainty as to whether an electron-capture (EC) to ground state 40Ar decay mode exists. Early compilations included this decay mode as inferred from an EC/𝛽+ calculated by Brosi and Ketelle, reported as a private communication in Engelkemeir (1962). However, a recent review by Min et al. (2000, GCA v64 p73) described this decay mode as "...unverified and its existence is questionable" and it was therefore not included. Here, we use the Fermi theory of beta decay with nuclear data from Bambynek et al., (1977, Rev. Mod. Phys. v49 p77) for 3rd order forbidden decays to estimate the 40K EC/𝛽+. Our estimate of ~200 is consistent with that of Brosi and Ketelle, the extrapolated log ft value, and other calculations from the literature. We validate this strategy by reproducing the EC/𝛽+ of 22Na, for which this ratio has been measured. Despite the lack of direct measurements, we find a high likelihood that the 40K EC to ground state decay mode exists and results in the production of radiogenic 40Ar, contributing as much as ~1% on the 40Ar-branch side. It is not clear, however, how best to incorporate a calculated quantity with a poor uncertainty budget and limited traceability into current best-estimates of the 40K decay scheme.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11D0116I
- Keywords:
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- 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRY;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 1094 Instruments and techniques;
- GEOCHEMISTRY