Equation of state of Al-bearing perovskite to lower mantle pressure conditions

Al2O3 is estimated to total 4 to 5 mol% in all mantle compositional models, and is believed to be incorporated into (Mg,Fe)SiO3-perovskite at lower mantle conditions. Using synchrotron X-ray diffraction, we have measured the 300 K equation of state of a perovskite with X-Al = Al/(Al+Mg+Si) = .077, up to 32 GPa. A least squares refinement of two independent data sets yields the following 3 equation of state parameters V-0 = 163.52(3) Angstrom (3), K-0 = 229(4) GPa, and K-0 = 2.5(4). These values allow us to assess the most probable substitution mechanism for aluminium into perovskite at pressure conditions of the shallow lower mantle. Al3+ is likely to be incorporated in place of Si4+ in the octahedral site of perovskite, hence requiring the creation of oxygen vacancies for charge balance. As a consequence, aluminous perovskites may have a high affinity for water. The enhanced compressibility of aluminous perovskite certainly has also strong geophysical and geochemical implications, as it may revive perovskite-rich lower mantle models.