Constraining the 40K decay constant with 87Rb-87Sr – 40K-40Ca chronometer intercomparison

Naumenko-Dèzes, Maria; Nägler, Thomas; Mezger, Klaus; Villa, Igor Maria (2018). Constraining the 40K decay constant with 87Rb-87Sr – 40K-40Ca chronometer intercomparison. Geochimica et cosmochimica acta, 220, pp. 235-247. Elsevier Science 10.1016/j.gca.2017.09.041

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A literature survey reveals that the K-Ar chronometer gives ages that are ca. 1% younger than U-Pb ages. This offset is generally attributed to an inaccurate 40K decay constant. Three geological samples selected from a shortlist of eight with known U-Pb ages were investigated using detailed petrological methods and subsequently the Rb-Sr and K-Ca chronometers in order (a) to evaluate if they meet the requirement of a geological history reflecting a “point-like” event (i.e. isochronous formation and subsequent ideal closure of chronometers) and (b) to narrow down the systematic uncertainty on the 40K decay constant by investigating the metrologically traceable K-Ca decay branch. Lepidolite of the Rubikon pegmatite, Namibia, was dated with Rb-Sr at 504.7 ± 4.2 Ma and the phlogopite and apatite from the Phalaborwa carbonatite complex, South Africa, yielded a Rb-Sr age of 2058.9 ± 5.2 Ma. Both Rb-Sr ages agree with published U-Pb ages. The Rb-Sr age of the late Archean Siilinjärvi carbonatite, Finland, records a later regional metamorphic event at 1869 ± 10 Ma. Only the samples from the Phalaborwa complex represent a “point-like” magmatic event and meet all the criteria to make them suitable for the 40K decay constant intercalibration. The Phalaborwa K-Ca isochron has a slope of 1.878 ± 0.012. Forcing the K-Ca isochron to coincide with the U-Pb and Rb-Sr ages gives one equation with two unknowns. Assuming that the branching ratio of the K-Ca branch, BCa, lies in the interval (k = 2) of all published references, 0.8925 < BCa < 0.8963, then the most reliable uncertainty interval (k = 2) for the total 40K decay constant, λtot, is calculated as 5.484 × 10−10 a−1 < λtot < 5.498 × 10−10 a−1. This confirms that the currently used IUGS recommendation is inaccurate.

Item Type:

Journal Article (Original Article)


08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology
08 Faculty of Science > Other Institutions > Emeriti, Faculty of Science

UniBE Contributor:

Nägler, Thomas; Mezger, Klaus and Villa, Igor Maria


500 Science > 550 Earth sciences & geology




Elsevier Science


[42] Schweizerischer Nationalfonds




Igor Maria Villa-Toscani

Date Deposited:

18 Jun 2019 13:52

Last Modified:

18 Jun 2019 13:52

Publisher DOI:





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