Phase relations and formation of chromium-rich phases in the system Mg4Si4O12–Mg3Cr2Si3O12 at 10–24 GPa and 1,600 °C

Abstract

Phase relations in the system Mg4Si4O12–Mg3Cr2Si3O12 were studied at 10–24 GPa and 1,600 °C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the knorringite–majorite system to derive a P–X phase diagram and synthesize garnets of a wide compositional range. Samples synthesized in the pressure range 10–14 GPa contain knorringite–majorite garnet and Cr-bearing pyroxene. With increasing Cr content in the starting materials, an association of knorringite–majorite garnet and eskolaite is formed. Garnets contain a significant portion of majorite (>10 mol%) even for a pure Mg3Cr2Si3O12 starting composition. Knorringite–majorite garnets were obtained in the pressure range from 10 to 20 GPa. With increasing pressure, the phase assemblages include Cr-bearing MgSiO3 akimotoite and MgSiO3 bridgmanite, as well as MgCr2O4 with calcium titanate structure, and stishovite. Single-crystal X-ray diffraction shows that the incorporation of Cr into the structure of garnet, as well as MgSiO3 akimotoite, and bridgmanite results in an increase in their unit cell parameters. Results of the experimental high-pressure investigation of the pseudo-binary system Mg4Si4O12–Mg3Cr2Si3O12 (SiO2–MgO–Cr2O3) may be applied to the origin of high chromium phases (mostly garnet) found as inclusions in peridotitic diamonds and formed in bulk rock compositions with high Cr/Al ratios in relation to the primitive mantle.