A mantle metasomatic injection event linked to late Cretaceous kimberlite magmatism

METASOMATISM of the lithospheric upper mantle by magmas and/or other fluids may induce significant changes in its chemical and mineralogical composition. It is not yet clear whether metasomatism takes place earlier than, concurrent with or subsequent to alkaline igneous activity and melt migration. Clues about the timing of metasomatism beneath the Kaapvaal craton in southern Africa are provided by metasomatized peridotites brought to the surface as xenoliths in young (80–95-Myr-old) group I kimberlites 1,2 : the question that arises is whether the metasomatic alterations of the xenoliths are related to the host kimberlites themselves or to earlier igneous events such as the group II kimberlite eruptions (120–150 Myr) 1,2 . Here we report a precise U–Pb age of 85±2 Myr for zircons in a veined and metasomatized harzburgite xenolith from Kimberley, which indicates that this particular style of metasomatism (MARID-related 3 ) is concurrent with the migration of the kimberlite magma that hosts the xenolith. This temporal link supports previous isotopic evidence 4 for a genetic link between the group I kimberlite magmatism and the accompanying metasomatism.