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

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 delta Zn-66 from 0.14 parts per thousand to 0.50 parts per thousand and from 0.29 parts per thousand to 0.58 parts per thousand, respectively; reconstructed whole-rock delta Zn-66 is from 0.24 parts per thousand to 0.54 parts per thousand, which is higher than typical mantle values (0.16-0.20 parts per thousand). Their heavy Zn- and light Mg- and O-isotope compositions (delta Mg-26 = - 1.1 parts per thousand to -0.14 parts per thousand, 6 18 0 = 2.3 parts per thousand to 4.9 parts per thousand) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low delta O-18 may be inherited from parental melts derived from low-delta O-18 mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high delta Zn-66 and low delta Mg-26 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 delta Zn-66 from 0.03 parts per thousand to 0.43 parts per thousand and garnet delta O-18 from 6 parts per thousand to 9.1 parts per thousand 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.