Phase Relations in the Model System SiO2–MgO–Cr2O3: Evidence from the Results of Experiments in Petrologically Significant Sections at 12–24 GPa and 1600°C

Abstract

The new results of an experimental study of the majorite MgSiO₃–magnesiochromite MgCr₂O₄ model section are discussed, and general topology of the SiO₂–MgO–Cr₂O₃ system is analyzed. Despite the absence of some petrogenic components (CaO, FeO, Al₂O₃, Na₂O, K₂O, and others) in this system, our study, performed in wide pressure range (10–24 GPa), allows us to consider all of the most important phase transformations (in this case, magnesium silicates and oxides) in the upper mantle, transition zone, and uppermost lower mantle. Addition of Cr shows the influence of a minor element on the phase transition parameters. New data on the solubility of Cr in deep minerals (garnet, olivine, wadsleyite, ringwoodite, and bridgmanite) were obtained, which allowed us to determine the influence of Cr on the structural patterns of the major mantle phases. It is shown that addition of 1 wt % Cr₂O₃ shifts the boundaries of phase transformations by 50 km (olivine/wadsleyite) and 10 km (wadsleyite/ringwoodite) to a lower-pressure domain in comparison with Cr-free systems. In a first approximation, the results of experimental study of phase relations in pseudobinary sections of the SiO₂–MgO–Cr₂O₃ system simulate the phase composition of the restitic part of the upper mantle, transition zone, and uppermost lower mantle under partial melting conditions. It is shown that the Cr concentration in mantle phases is significantly controlled by the Cr/Al ratio in the protolith.