Normal potassium, inherited argon in Zaire cubic diamonds

Abstract

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.