ALUMINUM DEPLETION IN KOMATIITES AND GARNET FRACTIONATION IN THE EARLY ARCHEAN MANTLE - HAFNIUM ISOTOPIC CONSTRAINTS

Hafnium isotopic compositions were measured in Al-depleted and Al-enriched komatiites from the 3450 Ma old Barberton greenstone belt, southern Africa. All samples have initial ε{lunate}Hf values close to zero. Such values are at variance with the strongly negative or positive values that should be observed if these rocks came from old garnet-depleted or garnet-enriched layers, such as may have formed during the solidification of an ancient terrestrial magma ocean. The garnet fractionation observed in komatiites probably took place during the melting event. © 1990.