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.

Naumenko-Dèzes, M., Nägler, T., Mezger, K., Villa, I. (2018). Constraining the 40K decay constant with 87Rb-87Sr – 40K-40Ca chronometer intercomparison. GEOCHIMICA ET COSMOCHIMICA ACTA, 220, 235-247 [10.1016/j.gca.2017.09.041].

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

Villa, IM
2018

Abstract

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.
Articolo in rivista - Articolo scientifico
40; K decay constant; Geochronometers intercalibration; K-Ca dating; Phalaborwa; Rb-Sr dating;
40K decay constant; Geochronometers intercalibration; K-Ca dating; Phalaborwa; Rb-Sr dating; Geochemistry and Petrology
English
2018
220
235
247
partially_open
Naumenko-Dèzes, M., Nägler, T., Mezger, K., Villa, I. (2018). Constraining the 40K decay constant with 87Rb-87Sr – 40K-40Ca chronometer intercomparison. GEOCHIMICA ET COSMOCHIMICA ACTA, 220, 235-247 [10.1016/j.gca.2017.09.041].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/220060
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