KOMATIITES: GEOCHEMISTRY AND GENESIS

Some important petrographic and chemical features of peridotitic komatiites (PK) and basaltic komatiites (BK) are reviewed. New chemical data (major elements and Zr, Nb, Rb, Sr, Y, Ba. Sc, V, Ni, Cr) are presented for samples from Barberton (South Africa), Belingwe (Rhodesia), Munro (Ontario) and Yilgarn and Pilbara (Australia). The PK are divided into two types. Those of type I, with very high CaO/Al2O3 (about 1.5) and low Al2O3/TiO2 ratios (about 11), are depleted in Al, heaw rare earth elements (HREE), V and Sc. We refer to these as aluminum-depleted peridotitic komatiites (ADPK) and their basaltic counterparts as ADBK. The type-komatiites from Barberton fall into this category. Type 2, which contains the bulk of komatiites, has CaO/Al2O3 ratios of about one (i.e., greater than chondritic), Al2O3/TiO2 ratios of about 20 (i.e., close to chondritic) and flat HREE patterns. We refer to these rocks as aluminium-undepleted (i.e., AUPK, AUBK). A sequential melting model involving Al retention by orthopyroxene might explain type 2, but garnet involvement seems necessary to explain type 1. Trace-element (particularly REE) data suggest that many high-Mg basalts (11-16% MgO) are primary melts and not fractionation products from PK. In particular, one group of basalts with characteristic quench textures (STB) have light REE enrichment, flat HREE and low abundances of Ti, Zr and, Y. These features probably reflect the nature of the basalt source, rather than subsequent differentiation. Discriminant diagrams such as FeO*/(FeO* + MgO) versus Al2O3 are of little use in differentiating tholeiitic from komatiitic basalts and serve only to separate fractionated basalts from more primitive basalts. Komatiites represent large-percentage partial melts of the mantle and require very high temperatures in the original source. This implies that they are initiated deep (about >= 400 km) in the mantle and rise owing to thermal instability. Post-Archean komatiites, although rare, are known to occur in the Proterozoic Cape Smith-Wakeham Bay province of northern Quebec. We do not include low-Ti, high-Me ophiolitic basalts in the komatiite group.