Melting Relations in the Model Pyrolite at 2.5, 3.0, 7.0 GPa and 1400–1800°C: Application to the Problem of the Formation of High-Chromium Garnets

Resumo

The study of partial melting of model pyrolite showed that garnets synthesized at 7 GPa within the temperature range of 1400–1800°C were characterized by an excess in Si content (>3 f.u.) and a stable admixture of Cr₂O₃, and thus are pyrope–majorite–knorringite solid solutions. An increase in the Cr/Al ratio in the starting composition results in an increase of Cr/Al in garnet. With increasing temperature, the concentration of Cr₂O₃ in restite from partial melting decreases, whereas the chromium content in the melt increases. This is accompanied by an increase in the Cr/Al ratio in all garnets (from the zones of restite and quenched melt). Estimates of the bulk compositions of restite produced via partial melting of model pyrolite at 2.5 and 3.0 GPa showed that the concentration of chromium in restite was higher than that in the starting material. All minerals from the restite zone are characterized by high chromium concentrations, since partial melting within the spinel-depth facies results in redistribution of chromium into restite. The results we obtained show that the origin of high-Cr garnets is related to the protolith with a high Cr/Al ratio formed as a residue from partial melting in the field of spinel stability and further removed into the garnet-depth facies.