Provenance studies from 40Ar/ 39Ar dating of mineral inclusions in diamonds: Methodological tests on the Orapa kimberlite, Botswana

Abstract

Previous studies indicate that clinopyroxene inclusions extracted from their host diamonds give 40Ar/ 39Ar ages approaching the time of source kimberlite eruption. This behaviour is attributed to diffusion of argon to the diamond/inclusion interface region during mantle residence, with subsequent loss of this component on cleaving of the diamond to release the inclusion(s). In this study, we investigate the potential of the 40Ar/ 39Ar inclusion dating method as a niche provenance tool. As diamondiferous kimberlites are uncommon igneous rocks, largely restricted to Archaean cratons and have limited age ranges, provenance studies of detrital diamond occurrences should provide unique information on craton erosional histories, palaeo-drainage evolution and related basin development. As a methodological test, 40Ar/ 39Ar step-heating and in vacuo crushing experiments were conducted on 50 eclogitic clinopyroxene inclusions extracted from diamonds from the ∼ 93 Ma Orapa kimberlite in Botswana. Low temperature steps and crushing experiments produced anomalously old ages (up to 2.98 ± 0.10 Ga), consistent with partial retention of pre-eruption (inherited) argon in sub-micron(?) defect sites. High temperature (fusion) steps yielded significantly younger ages, although only 35% are within error of the time of Orapa kimberlite eruption. However, 77% of results are within 50 Ma, and 92% within 100 Ma, of the Orapa eruption age. These results mandate that individual fusion ages be treated as maximum estimates of source kimberlite eruption ages, with analyses of several (> 6) inclusions required to date single-age detrital populations. For multi-source diamond deposits, only the youngest detrital population, or populations separated by > 100 Ma, will be resolvable. Despite these limitations, we conclude that the 40Ar/ 39Ar inclusion dating method is still capable of resolving key provenance questions, such as the source of detrital diamond deposits in southern Africa, where controversy exists as to whether the diamonds originated from proximal Permo-Carboniferous glacial deposits or distal Cretaceous/Jurassic kimberlites. Finally, a model is presented to account for the variable retention of pre-eruption argon in clinopyroxene inclusions extracted from their host diamonds. The proportion of this component retained by inclusions is considered dependent on interface porosity volume, partitioning of argon into defect structures, and the timing of stress fracture development around inclusions during entrainment to surface.