Subsolidus phase relations and perovskite compressibility in the system MgO-AlO_1.5-SiO₂ with implications for Earth's lower mantle

Experimentally determined phase relations in the system MgO-AlO_1.5-SiO₂ at pressures relevant to the upper part of the lower mantle indicate that Mg-silicate perovskite incorporates aluminum into its structure almost exclusively by a charge-coupled reaction. MgSiO₃-rich bulk compositions along the joins MgSiO₃-MgAlO_2.5 and MgSiO₃-MgAl₂O₄ crystallize assemblages of perovskite coexisting with periclase. MgO-saturated perovskites along these joins have ambient unit cell volumes consistent with those measured and calculated for aluminous perovskite along the charge-coupled substitution join, MgSiO₃-AlO_1.5. The compressibility of aluminous perovskite along the MgO-saturated joins is not anomalously low as predicted for oxygen-defect perovskites. The bulk moduli, however, are consistent with previous measurements made for aluminous perovskites along the charge-coupled substitution join. These results agree with first-principles calculations showing very limited stability of O-defects in Mg-perovskite at pressures and temperatures corresponding to lower mantle conditions, but are inconsistent with earlier experimental results showing unusually compressive aluminous perovskite. The maximum solubility of alumina in perovskite is ∼25 mol% along the MgSiO₃-AlO_1.5 join within the ternary MAS-system (i.e. pyrope composition), and the join is apparently binary. Although primitive mantle peridotite compositions are MgO-saturated and fall nearly on the oxygen vacancy join, alumina substitution into perovskite is expected to occur primarily by charge-coupled substitution throughout the lower mantle. The compressibility of aluminous perovskite in primitive mantle is expected to be only a few percent lower than for end member MgSiO₃ perovskite.