Trace element and isotope studies on veined, metasomatic and "MARID" xenoliths from Bultfontein, South Africa.

Concentrations of K, Rb, Sr, U, Pb and the REE as well as Sr, Nd and Pb isotope ratios are reported for separated mineral fractions from mantle-derived xenoliths showing textural and/or mineralogical evidence of metasomatism (e.g. veining, K-richterite or phlogopite) and from MARID xenoliths (Mica Amphibole Rutile Ilmenite Diopside, believed to be igneous). All samples are from the dumps of the Bultfontein kimberlite mine in Kimberley. Mineral phases analysed include mainly K-richterite, phlogopite, diopside and olivine. The study concerned the nature of the metasomatic processes and the character of the metasomatic fluid or liquid and that generating the MARID samples. REE and LILE concentrations vary greatly even within single mineral species (except diopside), but strong relative light REE enrichment is prevalent throughout, even in pre-metasomatic olivines. A single type of liquid or fluid may, however, have been responsible for the metasomatism in all samples and the generation of the MARID material, if a subsolidus reaction producing K-richterite from phlogopite is assumed. The liquid appears to have had kimberlitic affinities, and the metasomatism and generation of MARID material is probably younger than 200 Ma. The Sr and Nd isotopes form a pattern consistent with mixing of material from a slightly depleted mantle source (the metasomatising agent) with mantle material showing ancient enrichment in incompatible elements (typical of subcontinental lithosphere). K-richterite appears to be a sink for Pb and its absence or presence in old K-enriched mantle may possibly control correlations or anticorrelations between Sr and Pb isotope systems such as observed in volcanic rocks.