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The system MgO-Al₂O₃-Cr₂O₃ revisited

¹ Institute for Geology of Ore Deposits, 35 Staromonetny, Moscow 119017, Russia. Corresponding author, e-mail: girnis{at}igem.ru
² Institut für Mineralogie, J.W. Goethe-Universität, Senckenberganlage 28, D-60054 Frankfurt am Main, Germany
³ Institute of Mineralogy and Petrography, Koptyuga prosp. 3, Novosibirsk 630090, Russia
⁴ Vrjie University, Amsterdam, The Netherlands

New experimental data are presented on the partitioning of Cr and Al between spinel-picrochromite and corundumeskolaite solid solutions at 5.0 GPa and 1300, 1500°C and garnet-spinel-orthopyroxene equilibria at 3.0 and 4.0 GPa and 1000°C. The experimental products of Doroshev et al.'s (1997) experiments on mineral equilibria in the MASCr system were reanalysed in order to clarify the direction and mechanism of approach to equilibrium. Detailed analysis of mineral compositions was carried out by electron microprobe with high spatial resolution, which demonstrated complex zoning patterns of minerals. In experiments with pure pyrope in the starting material, garnet shows simple zoning with Cr-rich seams mantling relicts of aluminous garnet. In experiments with Cr-rich garnets in the starting mixture (Cr/(Cr+Al) = 0.3–0.8), more complex patterns were observed. Knorringiterich garnet cores are overgrown in sequence by Al-rich (X_Cr= 0.1–0.2) and then Cr-rich zones. The analytical data allowed us to establish the direction of approach to equilibrium for particular grains, which eliminated the path-looping problem. The results were compared to published experimental data on garnet, orthopyroxene, spinel, and eskolaite equilibria in the MAS and MASCr systems. It was shown that all the data can be described by a simple thermodynamic model, if the uncertainty of analytical results is taken into account.

Key-words: experiment, phase equilibrium, garnet, orthopyroxene, thermodynamic modeling.

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