Majorite fractionation and genesis of komatiites in the deep mantle

Abstract

Abstract Melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone. A model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid. Alumina-depleted komatiite, generally observed in the early Archean, can be generated by any degree of partial melting at depths from 450 to 650 km. This type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km. Alumina-enriched komatiite can be generated by remelting of the residual mantle after separation of alumina-depleted komatiite. Alumina-undepleted komatiite generally observed in the late Archean can not be formed by a low degree of partial melting of the primitive mantle at any depths. This type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km. As an alternative model, the alumina-depleted komatiite can be formed by a high degree of partial melting of the Archean upper mantle depleted in majorite. Such heterogeneity in the Archean upper mantle was generated by a global melting event succeeding the accretion of the earth, and erased by convection by the later Archean. Hf-isotope data seem to favour the former model of the petrogenesis of komatiite, i.e., single stage melting of the primitive mantle at various depths.