Ultrahigh-pressure phase relations in the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂

High-pressure phase relations in the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂ were investigated at pressures from 10 to 22 GPa and at a temperature of 1000°C. The experimental results indicate that the maximum solubility of enstatite (Mg₄Si₄O₁₂) in pyrope (Mg₃Al₂Si₃O₁₂) is 80 mol% enstatite at 15 GPa. At pressures between 15 and 19 GPa, the solubility is insensitive to pressure. At 19 GPa, garnet-ilmenite transformation first occurs in enstatite composition. At pressures above 19 GPa, the compositional range of the ilmenite solid solution expands toward pyrope composition with increasing pressure. Based on the present experiments, if pyrolite mantle is assumed, the pyroxene-garnet transformation appears at 350-400 km depth, and garnet + modified spinel and garnet + spinel phase assemblages become stable successively up to ∼ 580 km depth. At greater depths, garnet decomposes to ilmenite + garnet and hence the pyrolite mantle consists of ilmenite + garnet + spinel assemblage.