The diamond-poor nature of corundum-bearing eclogite and Karoo-type overprint of deep eclogite sources beneath the Kimberley region (Kaapvaal craton)

Decades of studies have shown that the petrogenesis of kimberlite-borne cratonic eclogite and pyroxenite xenoliths reflects the endemics of their crustal protoliths and local lithosphere evolution. Detailed investigations of the origin and metasomatic history of individual eclogite xenolith suites are thus required to understand how cratonic eclogite reservoirs - and their diamond inventory - evolve in the regional tectonomagmatic context. Here, we investigate a little-studied eclogite and pyroxenite xenolith suite from the Balmoral kimberlite in the Kimberley area of the Kaapvaal craton, which, like eclogite suites in neighbouring kimberlites, likely originated as subducted Archaean oceanic crust. Detailed petrographic observations and mineral major- and trace-element analyses, combined with published data for eclogite xenoliths and eclogitic inclusions in diamond, show that this sample suite records at least two distinct episodes of metasomatic overprint: (1) Metasomatism by a kimberlite-like melt caused a decrease in clinopyroxene jadeite component and garnet grossular component and imparted high MgO and Cr₂O₃ contents, recorded dominantly by pyroxenite xenoliths. Comparison to Kaapvaal kimberlites and lamproites confirms that the geochemical trends cannot be reconciled with bulk kimberlite-eclogite mixing but require precipitation of metasomatic clinopyroxene from the melt instead. This metasomatic style is recognised world-wide, and at Balmoral is notably restricted to the shallow lithosphere (110-150 km). (2) A distinct metasomatic event generated eclogites with extreme Y-HREE enrichment, at Balmoral restricted to the deep lithosphere (150-200 km). We propose that this enrichment style reflects phlogopite formation at the expense of garnet, with the liberation of these garnet-compatible elements to the metasomatic melt. This signature is identified in eclogite xenoliths both from early Cretaceous lamproite (Bellsbank) and late Cretaceous kimberlite (Balmoral, Kimberley) localities, tentatively ascribed to interaction with melts forming the Karoo large igneous province. Balmoral corundum-bearing eclogites derive from depths overlapping eclogites showing a Karoo-type overprint, which significantly diluted the Al₂O₃ content in the bulk rock and increased silica activity as gauged by the decrease in Al[IV] in clinopyroxene, thereby destabilising corundum.

Craton-wide, preserved corundum-bearing eclogites record diamond-stable ƒO₂ and pressure conditions, yet show little compositional overlap with inclusions in eclogitic diamond. This may reflect the low propensity of COH fluids to reach carbon saturation in this lithology. The preserved corundum-bearing eclogites have reconstructed bulk major-element compositions, which, combined with small to absent Eu anomalies, suggest protoliths representing deep oceanic crustal (>0.5 GPa) cumulates of two pyroxenes plus only minor plagioclase that contained a significant melt component. The identification of such deep hybridised crustal rocks may reflect higher mantle potential temperatures and the formation of thicker oceanic crust in the Archaean.