HR: 1340h
AN: MR23A-0195    [Abstracts]
TI: The Effect of Aluminum on the Perovskite to Post-Perovskite Phase Transition and Elasticity
AU: * Kiefer, B
EM: bkiefer@nmsu.edu
AF: Physics Department New Mexico State University, Box 30001, MSC 3D, Las Cruces, NM 88003-8001 United States
AU: Stixrude, L P
EM: stixrude@umich.edu
AF: Department of Geological Sciences; University of Michigan, 2534 C. C. Little Bldg., Ann Arbor, MI 48109-1063
AB: Seismology provides important constraints for compositional models of the earth's deep interior. These observations commonly show widespread lateral heterogeneity in the earth's lower mantle. However, the interpretation of seismic observations in terms of mantle composition and mantle dynamics relies critically on the knowledge of density and elasticity of the high pressure phases. It is thought that the earth's lower mantle phase assemblage is dominated by magnesium-rich perovskite that may contain 4-5 mol% Al$_{2}$O$_{3}$. Previous theoretical studies consistently predict that a direct substitution of 2 Al $->$ Mg + Si is favored at high pressures in Mg-perovskite. However, the effect of Al on the perovskite$->$post-perovskite phase boundary and the elasticity of the post-perovskite phase remain unknown. In order to address these issues we performed static first principle calculations for the direct substitution of Al in the post-perovskite phase. Our prelimnary results are based on Al-free and Al-bearing postperovskite with pyrope composition (25 mol% Al$_{2}$O$_{3}$). We find that the addition of Al has a large effect on the phase transition, it increases from 90 GPa for Al-free perovskite to 160 GPa for pyrope composition. Therefore even small amounts of Al may effect the phase boundary significantly and need to be included exploring the relation between the post-perovskite phase and D''. For the isotropic wave velocities we find at 120 GPa that 25 mol% Al$_{2}$O$_{3}$ reduces the compressional, shear, and bulk sound velocity, by ~2%, ~3.5%, and ~0.8%, respectively. Therefore Al seems to have only a modest effect on elasticity, however, the observed trend suggests that lateral variation in Al-content in the post-perovskite phase cannot account for the seimically observed anti-correlation between compressional and shear wave velocity in the lowermost mantle.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 1212 Earth's interior--composition and state (8105)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting