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Crystallization of megacrysts by carbonatitic metasomatism : evidence from the Muskox kimberlite, Nunavut, Canada Cone, Dylan


Low-Cr and high-Cr clinopyroxene, garnet, olivine, and ilmenite megacrysts from the Muskox kimberlite (Nunavut, Canada) have been analyzed for major and trace elements, as well as Sr, Nd and Pb isotopes in an attempt to constrain the debated origin of the kimberlitic megacryst suite. Samples display compositional overlap with respective phases in websterite, while clinopyroxene Sr-Nd-Pb isotope systematics reveal similarities with both websteritic clinopyroxene and metasomatic clinopyroxene in peridotites from the same kimberlite, in addition to whole-rock isotope data for the Muskox and neighboring Jericho kimberlites. All studied lithologies may represent the products of mixing between EM1 mantle, locally restricted relic Proterozoic enriched mantle and HIMU carbonatitic fluid. Equilibrium melts calculated using clinopyroxene trace element data reaffirm a carbonatitic affinity of the metasomatic agent. Thermodynamic modeling using the Deep Earth Water model shows that megacryst mineral compositions cannot be produced through metasomatism of mantle peridotite by H₂O-rich kimberlitic, asthenospheric or eclogitic fluids. The Sr-Nd-Pb isotope systematics argue against a strictly cognate relationship between Muskox megacrysts and the host kimberlite. Our findings rather suggest megacrysts and websterites represent products formed through regional metasomatism by carbonatitic HIMU fluids that predate kimberlitic magmatism.

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