Numerical Modeling of the Effects of Major Elements on the Solubility of Chrome–Spinel and a Likely Solution of the Problem of the Origin of Chromitite

Аннотация

The influence of variations in fo-, fa-, en-, fs-, di-, an-, and ab-components in a high-Mg basaltic melt on the topology of the spinellide liquidus was analyzed using the new model of the spinellide–melt equilibrium SPINMELT-2.0. It is shown that enrichment of the melt in the clinopyroxene component results in an increase in chromite solubility, whereas the plagioclase component leads to a decrease in chromite solubility. This effect may be of key importance under the conditions of gravitational shrinkage of cumulates accompanied by removal of the intercumulus melt and its upward infiltration. The successive re-equilibration of the infiltrating melt with cumulative rocks of different compositions may be expected. This allows us to assume the probability of transportation and subsequent accumulation of chrome–spinel at the post-cumulus stage in consolidation of layered intrusions. The nature of accumulation consists in the extraction of chrome–spinel into a melt enriched in the pyroxene component, which is followed by its release during the reaction of this melt with the feldspar matrix of protoanorthosite interlayers. The realism of the mechanism proposed is evident from the spatial relationship of the chromite layers with anorthosite in the Ram Island intrusion and in the Bushveld Complex.