Post-garnet transitions in the system Mg4Si4O12-Mg3Al2Si3O12 up to 28 GPa:phase relations of garnet, ilmenite and perovskite

Phase transitions in Mg3Al2Si3O12 garnet (pyrope) were examined at 23-28 GP a and 1600-2000 degrees C using a 6-8-type multianvil apparatus. It was fou nd that pyrope dissociated at 26.5-27 GPa and 1600-2000 degrees C into MgSi O3-rich perovskite solid solution and Al2O3-rich corundum solid solution wi th a slightly positive dP/dT slope. Perovskite solid solution containing 10 mol% Al2O3 was stabilized at about 26.5 GPa and 1600 degrees C, coexisting with corundum solid solution of 23 mol% MgSiO3. Powder X-ray diffraction of the single-phase aluminous perovskites in the s ystem MgSiO3-Al2O3 showed that these were orthorhombic and that the increas e in the b- and c-axes and a slight decrease in the a-axis accompanied incr easing Al2O3 content. Our data and other available X-ray data indicate that the unit cell volume of perovskite in the system MgSiO3-Al2O3 is best expr essed as V (Angstrom(3)) = 162.35 + 6.95x, where x represents the mol fract ion of Al2O3 in perovskite (0 less than or equal to x less than or equal to 0.25). Phase relations on the Mg4Si4O12-Mg3Al2Si3O12 join were examined at 20-27 G Pa at 1600 degrees C, and also at about 900 degrees C. A wide two-phase fie ld of garnet + perovskite and a relatively narrow field of ilmenite + garne t were delineated at 1600 degrees C. However, at about 900 degrees C at 21- 26 GPa, the stability fields of single-phase ilmenite and of ilmenite + gar net were greatly expanded. Thus, in the cool interior of descending slabs n ear 660 km discontinuity, there may be a significant depth interval over wh ich garnet gives way to ilmenite-bearing assemblages, before ultimate trans formation to perovskite takes place. (C) 2000 Elsevier Science B.V. All rig hts reserved.