Mineral chemistry of a zircon-bearing, composite, veined and metasomatisedupper-mantle peridotite xenolith from kimberlite

Zircon-bearing veins in a harzburgite xenolith from kimberlite have imposed Ca-metasomatism on the harzburgite wall rock, in addition to adding K, Fe, Ti and OH. The zircon, previously dated to have an age similar to that of the xenolith-hosting kimberlite, shows higher Y, Nb, Ba, REE, Th and U cont ents than other mantle-derived zircons. Peripheral alteration of the zircon to baddeleyite and zirconolite, and alteration of vein ilmenite to perovsk ite suggest reaction with an evolving carbonatitic kimberlite melt. The hig h Cr2O3 content (0.77 wt%) of the zirconolite extends the compositional ran ge of terrestrial zirconolite.