Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes

A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana ) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjecte d to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxe ne Cr-spinel increase at the expense of garnet and orthopyroxene with incre asing degrees of metasomatism. The most metasomatised xenolith is a wehrlit e. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but en riched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemi cal composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculat ions show that the modally metasomatised rocks became enriched in incompati ble elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Majo r elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metas omatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poo r environment. The data suggest that the xenoliths represent the mantle wal lrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and inc ompatible element-enriched component of this melt percolated into the wallr ock along a strong temperature gradient and caused the observed range of me tasomatism.