Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

We have investigated the effect of Al³⁺ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO₃-AlO_1.5 join with up to 25 mol% AlO_1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30-100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic ( Pbnm ) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al³⁺ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions ( V₀). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al³⁺ causes a mild increase in compressibility, with the bulk modulus ( K₀) decreasing at a rate of -67±35 GPa/X_Al. This decrease in K₀ is consistent with recent theoretical calculations if essentially all Al³⁺ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg²⁺ and Si⁴⁺. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al³⁺ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO₃-AlO_1.5-SiO₂ indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO₃-AlO_1.5-FeO_1.5-SiO₂ based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al³⁺ and Fe³⁺ substitutions. Primitive mantle with about 5 mol% AlO_1.5 and an Fe³⁺/(Fe³⁺+Fe²⁺) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al³⁺ and Fe³⁺ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.