An experimental study of the garnet-perovskite transformation in the system MgSiO3-Mg3Al2Si3O12

Phase transformations in the system MgSiO3-Mg3Al2Si3O12 have been studied at pressures between 22 and 28 GPa, at 1500 degrees C, using a multi-anvil apparatus. Majorite garnet with less than about 10 wt.% Al2O3 was found to completely transform into an orthorhombic perovskite structure over a pressure interval between 24 and 27 GPa, whereas pyrope garnet disproportionated into sub-aluminous perovskite plus corundum solid solutions at pressures greater than 26.5 GPa. Alumina contents in the perovskite structure are dependent on temperature, and increase from 11 wt.% at 1500 degrees C to 17 wt.% at 1800 degrees C, at a pressure of 27 GPa. The alumina content of perovskite also increases with increasing pressure, such that aluminous perovskite with the pyrope composition (25 mol% Al2O3) should be formed by about 30-40 GPa. These observations are consistent with our recent experimental results on a pyrolite composition, which demonstrated that no separate aluminous phase exists in the upper part of the lower mantle. The entire inventory of Al2O3 in pyrolite can be accommodated in solid solution in magnesium silicate perovskite.