Oxygen isotope variations in Cr-poor megacrysts from kimberlite

As radiogenic isotope compositions of Cr-poor megacrysts from kimberlite demonstrate that the megacrysts and kimberlite hosts are genetically related (megacrysts are likely deep-seated liquidus phases), the oxygen isotope compositions of Cr-poor megacrysts are indicative of the delta O-18 of the host kimberlites and their mantle source regions. Oxygen isotope ratios of garnet megacrysts from Group I kimberlites (from slightly depleted mantle sources) worldwide (North America, southern Africa, Australia) are restricted (delta O-18(VSMOW) = 5.24 parts per thousand, SD = 0.15, SE = 0.01, n = 121) and typical of phenocrysts in magmas derived from upper mantle with "normal" oxygen isotope compositions. No significant involvement of subducted oceanic crust is indicated in the genesis of Group I kimberlites. Garnet megacrysts from Group II kimberlites (from "enriched" mantle, and restricted to southern Africa) have anomalously high O-18/O-16 ratios (delta O-18(VSMOW) = 5.59 parts per thousand, SD = 0.18, SE = 0.02, n = 55). Similarities in equilibration temperature and major element composition between garnet megacrysts from Group I and II kimberlites rule out roles for these parameters in producing the marked difference in O-18/O-16 between these two suites. Instead, the high O-18/O-16 values of the Group II garnets suggests incorporation of anomalously heavy oxygen from subducted ocean floor material in the source region of Group Il kimberlites (as much as 30% if eclogite [with delta O-18 approximately 6.5 parts per thousand] with an ocean floor protolith is the contaminant, less if the source of anomalous oxygen is pelagic or terrestrial sediment with higher delta O-18). Actual samples of potential Group H kimberlite source rocks (e.g., mica/amphibole veined peridotites) or plutonic crystallization products of such kimberlites (e.g., mica-amphibolerutile-ilmenite-diopside rocks) should have elevated delta O-18 values, but no such material has been previously described in southern African xenolith suites. Copyright (C) 2001 Elsevier Science Ltd.