HR: 08:30h
AN: U31A-01    [Abstracts]
TI: Ab Initio Molecular Dynamics Calculations on the High Temperature and Pressure Properties of the Postperovskite Phase and the Effect of Al³⁺, Fe²⁺ and Fe³⁺ on its Elasticity and Stability
AU: * Brodholt, J P
EM: j.brodholt@ucl.ac.uk
AF: Dept. of Earth Science University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Stackhouse, S
EM: s.stackhouse@ucl.ac.uk
AF: Dept. of Earth Science University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Wookey, J
EM: jwookey@earth.leeds.ac.uk
AF: Dept. of Earth Science, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Kendall, M
EM: m.kendall@earth leeds.ac.uk
AF: Dept. of Earth Science, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Price, G D
EM: d.price@ucl.ac.uk
AF: Dept. of Earth Science University College London, Gower Street, London, WC1E 6BT United Kingdom
AB: We have used ab initio molecular dynamics to calculate the elastic properties of the MgSiO₃ postperovskite phase under the appropriate conditions of the lowermost mantle. In addition, we have calculated the properties of the FeSiO₃ and Al₂O₃ postperovskite end-members in order to evaluate the effect of Fe and Al on the stability and elastic properties of the postperovskite phase. We find the following: a) In contrast to perovskite itself, shear-wave anisotropy does not change appreciably with temperature. b) A transversely isotropic aggregate of postperovskite aligned with a vertical [010] can produce V_SH > V_SV of about 1 to 2%, however, this would require about 50% alignment of the grains to account for the seismic observations. A far greater degree of anisotopy (>15%) is found when the symmetry axis is [001]. This would require a much smaller amount of crystal alignment, but also a slip system that is not the intuitively obvious. c) The postperovskite phase has an anomalously low value for dlogV_S/dlogV_P of about 1. This is in contrast to perovskite (about 2.0) and values from seismological observations (2 to 3). d) We find that Al₂O₃ has little effect on the relative stabilities of perovskite and postperovskite. However, the Al₂O₃ postperovskite phase shows a remarkable amount of shear anisotropy. e) Calculations on Fe bearing perovskites and postperovskites suggest that Fe strongly stabilises postperovskite, in agreement with experiments. f) We also show how the postperovskite phase can account for the apparent strength and depth of the observed P- and S-wave D" discontinuity.
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 7207 Core and mantle
SC: Union [U]
MN: 2005 Joint Assembly