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Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle |
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| Authors: | MJ Walter A Kubo T Yoshino J Brodholt Y Ohishi |
| Institution: | a Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan b Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Rd., Bristol, BS8 1RJ, UK c Department of Geosciences, Princeton University, Princeton, NJ 08544, USA d Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA e Department of Earth Sciences, University College London, Gower St., London, WC1E 6BT, UK f Laboratoire Sciences de la Terre, ENS-Lyon, CNRS UMR 5570, 46 Allee d'Italie 69364 Lyon Cedex 7, France g SPring8/JASRI, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo, 679-5198, Japan |
| Abstract: | We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3-AlO1.5 join with up to 25 mol% AlO1.5. 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/XAl. 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-AlO1.5-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-AlO1.5-FeO1.5-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% AlO1.5 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. |
| Keywords: | perovskite equation-of-state substitution mechanisms defects lower mantle |
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