Kimberlite-related metasomatism recorded in MARID and PIC mantle xenoliths

Abstract

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) xenoliths are thought to be formed by intense “primary” mantle metasomatism. These rocks also display secondary features, such as cross-cutting veins and geochemical zonation of matrix minerals, which probably reflect later metasomatic events. To investigate the nature and origin(s) of these secondary features, 28 MARID and PIC xenoliths from southern African kimberlites and orangeites have been studied. MARID-hosted veins contain both carbonate and Ti-rich phases (e.g., titanite, phlogopite), suggesting that they formed by the infiltration of a carbonated silicate melt. Elevated TiO2 contents in MARID matrix mineral rims are spatially associated with carbonate-dominated veins, suggesting a genetic relationship between vein formation and geochemical zonation. Spongy rims around primary MARID and PIC clinopyroxene are depleted in Na2O and Al2O3 relative to their cores, possibly reflecting mineral dissolution in the xenoliths during ascent and emplacement of the entraining kimberlite. The preservation of compositional differences between primary and secondary phases in MARID and PIC xenoliths indicates that metasomatism occurred shortly before, or broadly coeval with, kimberlite/orangeite magmatism; otherwise, at typical mantle temperatures, such features would have quickly re-equilibrated. Increased Na2O in some mineral rims (e.g., K-richterite) may therefore reflect equilibration with a more Na-enriched primitive kimberlite melt composition than is commonly suggested. Vein-hosted clinopyroxene 87Sr/86Sri (0.70539 ± 0.00079) in one MARID sample is intermediate between primary clinopyroxene in the sample (0.70814 ± 0.00002) and the host Bultfontein kimberlite (0.70432 ± 0.00005), suggesting that vein minerals are derived from interactions between primary MARID phases and kimberlite-related melts/fluids. Sulfur isotope compositions of barite (δ34SVCDT = +4.69 ‰) and sulfides (δ34SVCDT = −0.69 ‰) in carbonate veins reflect equilibration at temperatures of 850–900 °C, consistent with sulfur-rich melt/fluid infiltration in the lithospheric mantle. In contrast, vein carbonate C-O isotope systematics (δ13CVPDB = −9.18 ‰; δ18OVSMOW = +17.22 ‰) are not typical of kimberlites or other mantle carbonates (δ13CVPDB = −3 to −8 ‰; δ18OVSMOW = 6 to 9 ‰), and may represent post-emplacement hydrothermal interactions of the cooling kimberlite with crustal fluids. These constraints suggest protracted metasomatism of MARID rocks shortly before and during entrainment by the host kimberlite.