Eclogite xenoliths from Kimberley, South Africa — A case study of mantle metasomatism in eclogites

Eclogite xenoliths from the Kimberley diamond mine dumps were studied for major, trace elements and oxygen isotopes. The suite consists of typical garnet and clinopyroxene rocks as well as kyanite-eclogites and olivine- or orthopyroxene-bearing samples. All samples have Al[6]/Al[4] ratios > 2 which indicate high equilibration pressures, i.e. in the eclogite stability field as opposed to pyroxenites. Most samples are modally and cryptically metasomatized and contain up to 10 vol.% phlogopite plus ilmenite and rutile. Trace element geochemical characteristics of phlogopite and Ti-oxides are similar to those described for metasomatized peridotite xenoliths from the Kaapvaal craton, and show that the eclogite xenoliths experienced the same metasomatic event. Oxygen isotopic values of the phlogopites plot with those for pristine mantle, and are taken as evidence that the metasomatic agent did not have an exotic oxygen isotopic composition. In contrast, the eclogitic garnets are significantly lighter in oxygen isotopes than pristine mantle. These values cannot have been caused by the metasomatism that produced phlogopite, but are a preserved feature of the eclogite protoliths. The occurrence of kyanite is a strong indication for a low pressure origin of the protoliths, This, together with low oxygen isotopic values, a range of Mg-numbers of 59.5 to 88.8 and overall high trace element abundances compared to other eclogite xenolith suites, is taken as evidence that the protoliths of the Kimberley eclogites were seawater-altered oceanic cumulates which were metamorphosed in eclogite facies conditions and later overprinted by a mantle-derived metasomatic event.