Geochemistry of Garnet Megacrysts from the Mir Kimberlite Pipe (Yakutia) and the Nature of Protokimberlite Melts

Abstract

The chemical compositions of garnets from a megacryst association of the Mir kimberlite pipe have been studied. By petrogenic elements, the garnet megacrysts can be classified as high-Ti and low-Cr pyrope. The megacryst TiO₂ contents of the Mir pipe correlate inversely with the MgO and Cr₂O₃ contents. Modeling of the composition of garnets through a fractional crystallization process showed that the most suitable composition of the melts parental for the garnets of the megacryst association is picrite. The composition of garnets crystallized from the kimberlite does not correspond to the composition of the natural garnets from the Mir pipe. The kimberlites contain less Ti, Zr, Y, and HREEs, but are more enriched with strongly incompatible elements (LREEs, Th, U, Nb, Ta, and Ba) than the model composition of the melt suitable for crystallization of the garnet megacrysts.