Abstract
Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.
Highlights
Kimberlites and orangeites are small-volume igneous rocks occurring in diatremes, sills and dykes
Orangeite melts appear to have originated from a refractory mantle that experienced subsequent metasomatism by H2O/CO2-rich fluids/melts enriched in incompatible elements
We have examined the composition of primary mineral inclusions in MARID phases and determined the mineral major and trace element concentrations, clinopyroxene Sr-Nd-Hf isotope ratios and zircon U-Pb-Hf-O isotopic compositions for two MARID samples from the Bultfontein kimberlite (Kimberley, South Africa; Fig. 1)
Summary
Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. We report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. A number of upper mantle xenolith types entrained by kimberlite and orangeite magmas exhibit these characteristics (see, for example, refs 11–13), which supports the widely held hypothesis that orangeite melts are sourced from the metasomatized mantle lithosphere[1,9,10,14,15] This interpretation is consistent with Sr-Nd isotope data, which indicate a long history of lithospheric mantle enrichment for the orangeite source (that is, radiogenic Sr and unradiogenic Nd isotopes)[3,16]. It is noteworthy that the veined peridotite composition proposed by Mitchell[1] is very similar to that of the MARID (mica-amphibole-rutile-ilmenite-diopside) suite of mantle xenoliths[18]; the only notable differences are the presence of Ti-phases and apparent absence of carbonates in MARID rocks
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