A group of 10 cubic diamonds from Zaire has been found1 to contain correlated concentrations of 40Ar and K which, interpreted as a whole-rock K–Ar isochron with the usual assumptions, yield the unreasonable age of 6.0 Gyr. The same age has also been determined2 by 40Ar–39Ar analysis of four additional diamonds from the same group. One explanation for these data is that the potassium in these diamonds is isotopically anomalous, specifically, enriched at least twofold in 40K relative to normal K, as might have resulted from preservation of cosmic-ray-induced spallation K produced in iron-rich planetesimals before accretion to the Earth. We have examined the isotopic composition of K in three diamonds from the same group, and report here that, within analytical uncertainties (about 1%), 40K is present in normal abundance, and the hypothesis of isotopically anomalous K is not supported. An alternative explanation is that the 40Ar in these diamonds is a trapped or 'excess' component, but this would require the unusual circumstance of a good correlation between 40Ar and K (for example, by incorporation of a fluid phase), but a lack of correlation between 40Ar and other noble gas species, including 36Ar.
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Podosek, F., Pier, J., Nitoh, O. et al. Normal potassium, inherited argon in Zaire cubic diamonds. Nature 334, 607–609 (1988). https://doi.org/10.1038/334607a0
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