Oxide, sulphide and carbonate minerals in a mantle polymict breccia: Metasomatism by proto-kimberlite magmas, and relationship to the kimberlite megacrystic suite

Polymict breccias entrained by kimberlites are mantle xenoliths comprising coarse-grained mantle minerals (porphyroclasts) and rock clasts, cemented together by ilmenite, rutile, phlogopite, olivine and minor sulphides. These unusual xenoliths are generally considered to result from ascending primitive/precursor kimberlite magmas that crystallised in the magma conduit at lithospheric mantle depths. To enhance our understanding of these processes, we investigated the oxide, sulphide and carbonate minerals of a new polymict breccia (DU-1) from the Bultfontein Dumps (Kimberley, South Africa). Xenolith DU-1 contains heterogeneous domains of ilmenite-rich breccia surrounded by spinel harzburgite wall rock. The breccia domains also host resorbed Cr-diopside porphyroclasts and occasional Fe-Ni-Co sulphides segregations. Ilmenite occurs as large (up to 5 cm), chemically zoned lenses, associated with minor rutile. The ilmenite has Hf isotope values (initial ε Hf = 2.1-3.0) in the range of South African Group I kimberlites and associated megacrysts. The ilmenite lenses host primary multiphase carbonate-rich, olivine-dominated, phlogopite-dominated and sulphide mineral inclusions. Carbonate-rich inclusions host abundant magnesite and dolomite, with subordinate kalsilite, phlogopite, alkali-carbonates, phosphates and chlorides. The occurrence of carbonate-rich inclusions suggests entrapment of a Ca-Mg-rich alkali-carbonate melt during ilmenite growth. However, geochemical modelling indicates that this melt was not parental to the ilmenite. Instead, it is suggested that the ilmenite (and other cementing phases) crystalised from ilmenite and the other cementing phases crystallised from a S-bearing Ti-Fe-K-rich ultramafic silicate melt, derived from an ascending proto-kimberlite melt, which was modified subsequently by wall rock assimilation and/or magma mixing, porphyroclast dissolution and crystal fractionation. The alkali-carbonate melt could be the residual liquid after these processes occurred. Alternatively, the ultramafic silicate and alkali-carbonate melts were produced by liquid immiscibility from a silicate-carbonate proto-kimberlite melt. The occurrence of ilmenite inclusions in sulphide segregations, and sulphide inclusions in ilmenite lenses is indicative of late-stage silicate-sulphide liquid immiscibility. The major and trace element concentrations, Hf isotope composition and mineral inclusion content of ilmenite are consistent with a genetic relationship between polymict breccia formation and the kimberlite megacryst suite. We propose that polymict breccias and megacrysts were produced at different depths by common silicate-carbonate magmas during the early stages of kimberlite magmatism.