The influence of low aluminum concentrations on the composition and conditions of crystallization of majorite–knorringite garnets: Experiment at 7.0 GPa and 1500–1700°C

Abstract

Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al₂O₃–Cr₂O₃–SiO₂ system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg₃Cr₂Si₃O₁₂ included chromiumbearing enstatite MgSiO₃ (up to 3.2 wt % Cr₂O₃) and eskolaite Cr₂O₃. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.