ORIGIN OF KIMBERLITES AND RELATED MAGMAS

Rare earth fractionation in kimberlites implies that they were produced by partial melting in the presence of residual garnet, in accordance with the widely held belief that kimberlites were formed by small degrees of partial melting of a garnet lherzolite lithology in the upper mantle. However, recent discoveries in some kimberlites of diamond xenocrysts containing syngenetic inclusions of majorite, and of xenoliths which originally contained majoritic garnet are suggestive of a deeper, transition zone origin for kimberlites. Experiments on a synthetic Group I kimberlite were carried out using an MA-8 apparatus to evaluate this possibility. At 16 GPa and 1650-degrees-C, majorite garnet (13% Al2O3) and beta-M2SiO4 crystallize together on the liquidus, showing that this kimberlite magma could have been produced by a small degree of partial melting of a majorite + beta-M2SiO4 (or gamma-M2SiO4) assemblage in the transition zone (400-650 km). However, the first appearance of garnet well below the liquidus at 10 GPa implies that this typical kimberlite composition could not have been produced by a small degree of partial melting of garnet peridotite at depths shallower than 300 km, and casts doubt on conventional models of kimberlite petrogenesis. Isotopic, trace element and geochemical similarities imply a genetic relationship between kimberlites and ocean island basalts (OIBs). However, kimberlites were derived from a source possessing a higher Mg-number, and lower Na2O, Al2O3 and CaO contents than the OIB source. It is proposed that the ultimate source regions both of kimberlites and OIBs lie in the transition zone, in a boundary layer comprised of mixed domains of subducted former harzburgite and aesthenospheric pyrolite. The boundary layer was refertilized by partial melts derived from garnetite (former subducted oceanic crust) trapped on the 650 km discontinuity.