Zn-, Mg- and O-isotope evidence for the origin of mantle eclogites from Roberts Victor kimberlite (Kaapvaal Craton, South Africa)

Abstract

Abstract We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have δ66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock δ66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16–0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (δ26Mg = −1.1‰ to −0.14‰, δ18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low δ18O may be inherited from parental melts derived from low-δ18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high δ66Zn and low δ26Mg reflect diffusion-driven Zn-Mg–isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock δ66Zn from 0.03‰ to 0.43‰ and garnet δ18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.