Archean subduction recorded by Re-Os isotopes in eclogitic sulfide inclusions in Kimberley diamonds

Eclogitic sulfide minerals encapsulated in diamonds originating from the de epest part of the continental mantle keel beneath the Kaapvaal craton, sout hern Africa, and brought to the surface by the Kimberley kimberlites, show low Ni and Os contents and high Re/Os ratios characteristic of a basaltic p rotolith. The sulfide inclusions with the lowest Os contents give late Arch ean single grain absolute ages while those with higher Cls contents yield a well-constrained 2.9 Ga isochron age and radiogenic initial Os isotope com position (gamma (o), = +45). This indicates a significant time gap between basaltic precursor generation and eclogitic diamond crystallization, consis tent with extended residence in a near-surface environment prior to subduct ion associated with accretion of the Kimberley block to the rest of the cra ton and subsequent diamond formation. These results suggest that subduction -related crustal recycling was already a viable process during continent fo rmation in the middle Archean and may have been implicated in eclogitic dia mond formation ever since. (C) 2001 Elsevier Science B.V. All rights reserv ed.