Komatiites and other high-magnesia lavas: some problems

Abstract

Criteria for the recognition of high-magnesia liquids, among which the absence of phenocrysts is the most important, are discussed. Some high-magnesia lava sequences are strongly porphyritic but it can be demonstrated that their character is not due to the accumulation of ferromagnesian phenocrysts in normal basaltic magmas. The term primitive porphyritic magma is introduced to describe the magmas from which such sequences crystallize. Possible origins of primitive porphyritic magmas include advanced crystallization of high-magnesia liquids without loss of phenocrysts. The occurrence of Phanerozoic high-magnesia lavas associated with continental break-up is described and comparison is made with Archaean komatiite vulcanism. Low levels of incompatible elements are characteristic of the Archaean rocks but high CaO/Al 2 O 3 is not a specifically Archaean feature. Phanerozoic liquids with MgO much above 20 % have yet to be identified but may possibly have existed. Major-element data for komatiites are discussed with a view to the constraints they put on the composition of the source material. Several interpretations, which also have widely varying implications for depth of origin and degree of melting, are presently possible. A new model involving the complete mobilization of source material after a comparatively low degree of partial melting is presented. Bulk compositions of magmas produced lie on a mixing line between the composition of the source and the composition of the liquid fraction at the moment of mobilization.