CHROMITE IN KOMATIITES, II. MODIFICATION DURING GREENSCHIST TO MID-AMPHIBOLITE FACIES METAMORPHISM

Abstract

Chromite compositions in komatiites are influenced by metamorphic processes, particularly above 500°C. Metamorphosed chromite is substantially more iron rich than igneous precursors, as a result of Mg–Fe exchange with silicates and carbonates. Chromite metamorphosed to amphibolite facies is enriched in Zn and Fe, and depleted in Ni, relative to lower metamorphic grades. Relative proportions of the trivalent ions Cr3+, Al3+ and Fe3+ are not greatly modified by metamorphism up to lower amphibolite facies, although minor Fe3+ depletion occurs during talc–carbonate alteration at low temperature. Significant Al is lost from chromite cores above 550°C, as a result of equilibration with fluids in equilibrium with chlorite. Elevated Zn content in chromite is restricted to rocks with low (metamorphic) Mg/Fe ratios, and is the result of introduction of Zn during low-temperature alteration, with further concentration and homogenization during prograde metamorphism. Cobalt and Mn also behave similarly, except where carbonate minerals are predominant in the metamorphic assemblage. Chromite at amphibolite facies is typically extensively replaced by magnetite. This is the result of incomplete metamorphic reaction between chromite and chlorite-bearing silicate assemblages. Magnetite compositions at the inner chromite–magnetite boundary are indicators of metamorphic grade.