Enrichment processes in garnet-bearing mantle xenoliths from Kimberley pipes (South Africa)

Eight garnet-peridotites (5 harzburgites and 3 lherzolites) and two garnet-bearing ol-orthopyroxenites from Bult-fontein kimberlite dumps (Kimberley pipes, South Africa) were studied. They present protogranular textures with well-equilibrated primary mineralogical assemblage, consisting of large olivine (3-5 mm), orthopyroxene (3-5 mm), garnet (2-6 mm), phlogopite (up to 1 mm), and smaller clinopyroxene (2-3 mm), and spinel (up to 0.5 mm) crystals. Three types of secondary textures, superimposed on the primary paragenesis, were distinguished on the basis of geometric relationships and relative proportions of primary and secondary phases. Type A is characterised by the development of reaction coronas around garnets, which beside a dark keliphitic rim, consist of small subeuhedral crystals of phlogopite (up to 0.05 mm), spinel (100-400 μm) and clinopyroxene (0.05-0.1 mm). Type B is characterised by reaction areas around orthopyroxene. The secondary assemblage is constituted by small grai...