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Examination of heavy mineral concentrates from several well‐known eclogite‐dominated kimberlites (Roberts Victor, Bobbejaan, Zagadochnaya) has revealed that the upper mantle sampled by these kimberlites was dominantly peridotite, with only a small amount of eclogite (3–15% by volume). In the case of Group II (micaceous) kimberlites in southern Africa (e.g., Roberts Victor), it is proposed that garnet peridotites were metasomatized (phlogopitized) within the upper mantle prior to xenolith incorporation. This caused structural weakening, with the result that most of the garnet peridotites disaggregated during kimberlite ascent and eruption, leaving an eclogite‐dominated xenolith suite. The amount of eclogite in the upper 200 km of the subcontinental upper mantle is concluded to be perhaps <1% by volume overall. Support for models proposing a significant amount of eclogite at deeper levels or beneath ocean basins must be sought from evidence other than mantle‐derived xenoliths.

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The anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor. Recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons. Early crustal growth is thought to have been enhanced by the addition of slab-derived magmas, leaving an eclogite residuum in the upper mantle beneath the cratons. But the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean-floor basalt and intermediate-stage subduction-zone eclogites, and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean-floor basalts. We present here the oxygen isotope composition of eclogitic mineral inclusions, analysed in situ in diamonds using an ion microprobe/secondary ion mass spectrometer. The oxygen isotope values of coesite (a polymorph of SiO2) inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle, but are typical of subduction-zone meta-basalts, and accordingly provide strong support for the link between altered ocean-floor basalts and mantle eclogite xenoliths.

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