Denis R. Bell

Fine-grained pyroxenites from the Gansfontein kimberlite, South Africa: Evidence for megacryst magma-mantle interaction

The Gansfontein kimberlite contains a suite of fine-grained xenoliths dominated by orthopyroxene, and containing ilmenite, phlogopite, and occasional garnet, with minor quantities of olivine and sulphide. Lamellar intergrowths of orthopyroxene and ilmenite were observed in one sample. The fine grained orthopyroxenite assemblages were observed as discrete xenoliths, as a vein in lherzolite, and as a zoned margin surrounding a megacrystic dunite. The minerals are characterized by intra- and inter-grain chemical heterogeneity, but are on the whole compositionally similar to those in the abundant, highly evolved Cr-poor megacryst suite at Gansfontein. However, they differ to varying degrees from megacrysts in the concentration of minor elements such as Cr, Al and Ti. Mineral compositions in a pyroxenite vein in lherzolite are higher Cr and Mg#, and lower in Fe3+ than in the discrete fine-grained pyroxenites, indicating chemical interaction with peridotite. A single zircon-bearing mica-clinopyroxenite has mineral compositions similar to MARID xenoliths. Fine-grained orthopyroxenites, recognized previously from the Weltevreden and Mzongwana kimberlites and interpreted as rapidly crystallized magmas, are here suggested to result from a reaction between megacryst magma and solid mantle peridotite. Mica-clinopyroxenite may represent the liquid end-product of this reaction. Chemical and modal differences of orthopyroxenites from megacrysts result from reaction with peridotitic components, lack of buffering by typical megacryst mineral assemblages, and possibly shallower origins. Textures and fine-scale chemical disequilibrium indicate that reaction postdates some episodes of megacryst formation and was probably underway when the xenoliths were sampled by ascending kimberlite. Orthopyroxene-garnet thermobarometry indicates an origin of one Gansfontein pyroxenite at ~1215°C and ~3.3 GPa, similar to the locus of megacryst crystallization under East Griqualand. The Gansfontein pyroxenites indicate interaction of magmas at shallow levels within the subcontinental lithospheric mantle and provide evidence for melt-peridotite reaction that includes a possible reaction relationship between Ti-rich melt and olivine. Similar phase relations and processes may also play a role in megacryst petrogenesis, for which these pyroxenite xenoliths could represent a small-scale, initial stage analogue. The liquid end-products of such reaction may be important components of kimberlites and mantle metasomatic fluids.