Komatiite genesis: insights provided by Fe-Mg exchange equilibria

Abstract

Two possible models are available for the generation of komatiitic melts from upper mantle sources: (1) large extents of melting at relatively low pressures and (2) low extents of pseudoinvariant melting at high pressures. For the partial melting of olivine-dominated upper mantle and generation of komatiitic magmas, the [Mg]-[Fe] diagram calculated from olivine-melt K_D data is potentially useful in estimating the physical conditions of melting and in evaluating Fe/Mg ratios of mantle sources. Application of this diagram to a suite of komatiitic amphibolites derived by different extents of melting of similar sources from the Kolar Schist Belt, south India, indicates that (1) their magmas were derived at pressures ranging from 3 to 7 GPa by different extents of melting; (2) melt generation occurred in adiabatically rising deep mantle diapirs; and (3) diapiric mantle sources had higher Fe/Mg ratios than the undepleted Lesotho garnet lherzolite (PHN 1611). Komatiitic rocks from other Archean terranes in the [Mg]-[Fe] diagram suggests a range of pressures and degree of melting that agree with those proposed from high pressure experimental studies. Although some komatiitic rocks have geochemical features and conditions of melting that may require residual garnet in the magma source regions, such geochemical features can also be inherited from sources without much residual garnet. This is possible if the sources had undergone high pressure melt addition at the time of melting.