Low-Cr and high-Cr clinopyroxene, garnet, olivine and ilmenite megacrysts from the Muskox kimberlite (Canada) were analysed for major and trace elements, as well as Sr, Nd and Pb isotopes. Samples display compositional overlap with respective phases in websterites, and clinopyroxene isotope systematics reveal similarities to both websteritic and metasomatic clinopyroxene in peridotites from the same kimberlite, in addition to Muskox and Jericho kimberlites. All lithologies may represent the products of mixing between EM1 mantle, relict Proterozoic enriched mantle and HIMU carbonatitic fluid. Equilibrium melts calculated from clinopyroxene trace element data using experimental distribution coefficients for feasible proto-kimberlitic melts yield a range of metasomatic agents. Conclusion on the carbonate-bearing nature of the metasomatism is based on the presence of a HIMU isotopic signature and results obtained from thermodynamic modelling using the Deep Earth Water model. Modelling was carried out with three fluid compositions; asthenospheric, eclogitic and kimberlitic. Our results suggest mineral compositions analogous to megacrysts cannot be produced by metasomatism of mantle peridotite by carbonate-free hydrous fluids. Isotope systematics argue against a strictly cognate relationship between megacrysts and their host kimberlite, instead suggesting that megacrysts and websterites may represent products of regional metasomatism by carbonatitic HIMU fluids shortly predating kimberlite magmatism.
Supplementary material: A table containing parameters used in isotopic mixing models is available at https://doi.org/10.6084/m9.figshare.c.5255825