U–Pb, Rb–Sr (ref. 1) and Sm–Nd systematics2 in inclusions in diamonds suggest strongly that some diamonds are at least 2.0 Gyr old. Such antiquity is also inferred for some diamonds from their very primitive helium isotopic composition3. However, there has been almost no direct radiometric dating of diamonds, except for conventional K–Ar dating4,5, the results of which are questionable due to the possible presence of excess 40Ar. To avoid this problem, we have applied a K–Ar isochron dating method6 to ten diamonds from Zaire. The experimental data show good linear correlations in both the 40Ar–K and 40Ar/36Ar–K/36Ar diagrams. These correlations must reflect either an isochron-type relationship or the trapping of an unknown component in the diamonds. In the former case the anomalously high age (6.0 Gyr) casts doubt on the assumption of uniformity in the isotopic abundance of 40K.
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Zashu, S., Ozima, M. & Nitoh, O. K–Ar isochron dating of Zaire cubic diamonds. Nature 323, 710–712 (1986). https://doi.org/10.1038/323710a0
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