Phase relations in the model pyrolite at 2.5, 3.0, 7.0 GPа and 1400–1800°c: evidence for the formation of high-chromium garnets

Based on study of partial melting in the model pyrolite, it is shown that garnets synthesized at 7.0 GPa in a temperature range of 1400–1800°C are characterized by an excessive Si content (in relation to 3 f.u.), stable admixture of Cr2O3, and, thus, represent a solid solution of the pyrope–majorite–knorringite composition. Increase in the Cr/Al value in the starting composition results in increase of this ratio in garnet. With increasing temperature, the concentration of Cr2O3 decreases in restite and increases in melt. Cr/Al increases in all garnets from the zone of restite and from the quenched melt aggregate. Estimates of the bulk compositions of restite formed by partial melting of the model pyrolite at 2.5 and 3.0 GPa show that the concentration of Cr in it is higher than that in the starting composition. All minerals from the zone of restite are characterized by the high Cr concentrations, and upon partial melting in the spinel-depth facies, Cr is redistributed to restite. Our results show that the formation of high-chromium garnets relates to the protolith with the high Cr/Al value formed as a residue from partial melting in the spinel-depth facies and further transported to the garnet facies.