Melting in an Archaean mantle plume: heads it's basalts, tails it's komatiites

THE lower part of most Archaean greenstone sequences is dominated by interlayered basaltic and komatiitic (ultrabasic) flows. These two magma types are petrologically and geochemically distinct, yet they display a close spatial and temporal association. The origin of the anomalously high-temperature komatiitic liquids has been much debated because of the implications for the thermal structure and composition of the Archaean mantle. Here we argue that both the basalts and komatiites are produced by a starting thermal plume rising in a warmer Archaean mantle. Fluid-dynamics studies show that a starting plume consists of a hot axial jet, capped by a large head into which cooler surrounding mantle is entrained. Our calculations for such a flow indicate that komatiites could form by melting in the high-temperature axis of the plume and basalts by melting in the cooler head. The sudden onset and limited duration of the basalt/komatiite sequences, seen in the greenstone belts of Western Australia and elsewhere, are explained by this model.