KOMATIITE GENESIS - INSIGHTS PROVIDED BY FE-MG EXCHANGE EQUILIBRIA

Two possible models are available for the generation of komatiitic mel ts from upper mantle sources: (1) large extents of melting at relative ly low pressures and (2) low extents of pseudoinvariant melting at hig h 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 ph ysical conditions of melting and in evaluating Fe/Mg ratios of mantle sources. Application of this diagram to a suite of komatiitic amphibol ites derived by different extents of melting of similar sources from t he Kolar Schist Belt, south India, indicates that (1) their magmas wer e derived at pressures ranging from 3 to 7 GPa by different extents of melting; (2) melt generation occurred in adiabatically rising deep ma ntle diapirs; and (3) diapiric mantle sources had higher Fe/Mg ratios than the undepleted Lesotho garnet Iherzolite (PHN 1611). Komatiitic r ocks from other Archean terranes in the [Mg]-[Fe] diagram suggests a r ange of pressures and degree of melting that agree with those proposed from high pressure experimental studies. Although some komatiitic roc ks have geochemical features and conditions of melting that may requir e residual garnet in the magma source regions, such geochemical featur es can also be inherited from sources without much residual gamet. Thi s is possible if the sources had undergone high pressure melt addition at the time of melting.