In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton

A subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development. However, eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin (e.g., garnet d18O-values and mineral 87Sr/86Sr compositions). To assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond-forming and other possible metasomatic fluids, we have conducted a multitechnique in situ study of a diamondiferous eclogite xenolith from the Roberts Victor kimberlite, S. Africa. Using SIMS we provide the first texturally-controlled in situ measurements of d18O-values in eclogitic garnet in close proximity to diamond. Garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from Mg# = 75.8–79.2; grossular proportions = 8.05–10.14 mol.%, and omphacitic pyroxene has Jd13–24 and Mg# = 86.6–90.0. Rare earth element patterns of minerals across the xenolith, including grains close to diamond, are typical LREE-depleted garnets and markedly LREE-enriched pyroxenes. These silicate minerals also record detectable intra- and inter-grain LREE abundance variations. Clinopyroxenes of the studied xenoliths show HFSE and Sr abundance variations that are decoupled from LREE contents and major-element variations. Mineralogical constraints and bulk-rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small-volume (<<0.03 wt.%) infiltration of metasomatic fluid(s) potentially linked to ancient diamond evolution. Intra-grain major-element, LREE and HFSE variations in clinopyroxene resulted from latestage metasomatism. Oxygen isotope compositions in garnet are decoupled from all major- and trace-element variations, with garnet d18O-values being uniform across the xenolith in a wide variety of textural settings. Garnet d18O-values of +6.5 ± 0.2 ‰ are higher than the mean (+5.19 ± 0.26 ‰) of the mantle garnet range (+4.8–5.5 ‰)