Mesozoic to Paleoproterozoic diamond growth beneath Botswana recorded by Re-Os ages from individual eclogitic and websteritic inclusions

Abstract

Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics.Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.