Constraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene
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The trace element compositions of igneous augite preserved in metamorphosed komatiite samples from the lower Komati Formation, Barberton Mountainland, South Africa (3.49 Ga), were analyzed in order to estimate the pre-metamorphic bulk composition of the samples. The chemical effects of metamorphism are evaluated by calculating the trace element (TE) composition of a hypothetical augite in equilibrium with the bulk sample composition, and comparing it with ion probe analyses of the cores of augite crystals preserved in that sample. Differences between the calculated and measured composition of the augite indicate chemical gain or loss from the bulk sample during metamorphism. The data indicate that the rare earth elements (REE) and Y were immobile during metamorphism. Immobility is also indicated for Ti and Zr, though chromite fractionation makes this conclusion less firm. Sr and Eu were highly mobile (>50% loss) in all samples studied. Because the REE were immobile, the light REE (LREE) enrichments [(La/Sm)n>1] present in some Barberton komatiite samples are a primary igneous feature, and not the result of post-emplacement metamorphic processes. The variations in (La/Sm)n correlate with Ti/Zr in a strikingly similar manner to variations present in modern boninites, which are high Mg, hydrous subduction zone magmas. Plume-based models that appeal to majorite garnet fractionation cannot produce the low Ti/Zr of komatiites or their wide variation in La/Sm. Thus, we propose that a subduction origin for the Barberton komatiites and basaltic komatiites is more consistent with the available trace element data.
KeywordsSubduction Chromite Subduction Zone Trace Element Composition Trace Element Data
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