SPINMELT-2.0: Simulation of Spinel–Melt Equilibrium in Basaltic Systems under Pressures up to 15 Kbar: III. The Effects of Major Components in the Melt on the Chrome-Spinel Stability and a Possible Solution of the Problem of Origin of Chromitites

The effect of variations in the fo, fa, en, fs, di, an and ab components in high-Mg basaltic melts on the topology of the spinel liquidus was quantified by conducting simulations with the new SPINMELT-2.0 model for the spinel–melt equilibrium. It has been established that enrichment of the melt in pyroxene components leads to an increase, and conversely, enrichment in plagioclase and olivine components, to a decrease in the solubility of chromite. This effect can be important at the gravitational compaction of cumulates accompanied by the extraction of intercumulus melt and its infiltration upward. In this case, it is reasonable to expect sequential reequilibration of the infiltrating melt with cumulate piles of different composition. This suggests that Cr-spinel can be transferred and concentrated again during the postcumulus solidification stage of layered intrusions. The concentrating process involves Cr-spinel extraction into the melt enriched in pyroxene components and the subsequent redeposition of the mineral during when this melt reacts with the feldspar-rich matrix of proto-anorthosite layers or horizons enriched in olivine. Some of these layers and horizons may be produced by additional injections of more primitive magma. The realism of the proposed mechanism is evident from the well-known spatial relations between the chromite layers with anorthosites and dunite–harzburgites of the Bushveld complex.