Crystallization of megacrysts from protokimberlitic fluids: Geochemical evidence from high-Cr megacrysts in the Jericho kimberlite

We report petrological, geochemical and isotopic data for low-Cr and high-Cr suite megacrysts from the Jericho kimberlite (N. Canada). The megacrysts comprise garnet, clinopyroxene, orthopyroxene and olivine; ilmenite is present only in the low-Cr suite. The megacrysts occur as individual crystals or intergrown grains of the same or different megacryst minerals showing partial recrystallization. Compositional zoning, textures of polycrystalline megacrysts and a lack of extended MgO or SiO2 trends suggest megacrysts formed metasomatically shortly before the kimberlite emplacement. Fractionated REE patterns and radiogenic Hf compositions of Cr-rich megacrysts, as compared to low-Cr megacrysts elsewhere, imply a lower fluid/wall rock ratio than that characteristic of magmatic processes. Rb–Sr, Sm–Nd and Lu–Hf isochron ages for the megacrysts are similar to the eruption age of the kimberlite and represent blocking of the isotope systems at the eruption. Trace element patterns of melts buffered by megacrystal clinopyroxene and garnet were computed using experimental partition coefficients for silicate-carbonate high-pressure high-temperature melts. Our data suggests that the melt equilibrated with the megacrysts was kimberlitic as it has trace elements and Hf and Nd isotopic compositions of the host kimberlite. Sr isotopic range of the Jericho kimberlite can be modelled by 5–10% contamination of local crust such as limestone or granite, into a magma that had an initial unradiogenic Sr isotope composition similar to that of the megacrysts.