HR: 1340h
AN: MR43A-0875    [Abstracts]
TI: Ab Initio Molcular Dynamics Study of the High-Temperature Elastic Constants of hcp-Iron at Inner-Core pressures.
AU: * Gannarelli, C M
EM: che.gannarelli@ucl.ac.uk
AF: Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Vo\v{c}adlo, L
EM: l.vocadlo@ucl.ac.uk
AF: Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Alf\`{e}, D
EM: d.alfe@ucl.ac.uk
AF: Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Alf\`{e}, D
EM: d.alfe@ucl.ac.uk
AF: Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Gillan, M J
EM: m.gillan@ucl.ac.uk
AF: Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Price, G D
EM: d.price@ucl.ac.uk
AF: Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AB: It is well established that the inner core exhibits significant anisotropy with compressional P-wave velocities which are \(\sim 3\)~% faster along the polar axis than in the equatorial plane. Interpretation of this seismic anisotropy is hampered by lack of clear data on the physical properties of core phases at simultaneously high pressures and high temperatures. Although it is well known that there are light elements alloyed to iron in the inner core, even definitive results for the properties of pure iron at core conditions are proving elusive. Experimental groups have put an enormous effort over the last 10--15 years into obtaining the properties of pure iron at elevated pressures and temperatures, but above relatively modest \(P\) & \(T\) there is much uncertainty in the results. Theoretical calculations at inner core conditions on the hexagonal-close-packed phase of iron are achievable, but even here there are major uncertainties on the physical properties and the nature of the elastic anisotropy. In particular, results from previous theoretical calculations (Steinle-Neumann et al., 2001), based on the approximate ``particle in a cell'' method, suggest that the elastic properties of hcp-Fe change dramatically as a function of temperature. However previously we have shown these ``particle in a cell'' method results do not accurately describe the high \(P\), \(T\) behaviour of Fe (Gannarelli et al, 2003). Here we present the first \textit{ab initio} molecular dynamic simulation of the finite temperature elasticity of hcp-iron at core conditions. We conclude that again the previously reported (Steinle-Neumann et al., 2001) simulations of the high T behaviour of hcp Fe are in error, and we find that the sense of the elastic anisotropy at high \(T\) is essentially similar to its athermal behaviour. This supports the view originally expressed by Stixrude and Cohen (1995) that if the inner core anisotropy is associated with hcp Fe, then it is consistent with preferential alignment of Fe crystals with their \(c\)-axes closely aligned parallel to the rotation axis of the Earth. Steinle-Neumann, G., Stixrude, L., Cohen, R.E. and G\"{u}lseren O. (2001). ``Elasticity of iron at the temperature of the Earth's inner core'', Nature, \textbf{413}, 57. Gannarelli, C.M.S., Alf\`{e}, D. and Gillan, M.J. (2003). ``The particle-in-cell model for \textit{ab initio} thermodynamics: implications for the elastic anistropy of the Earth's inner core'', Phys. Earth Planet. Inter., \textbf{139}, 243. Stixrude, L. and Cohen, R.E. (1995). ``High-pressure elasticity of iron and anisotropy of Earth's inner core'', Science, \textbf{267}, 1972.
UR: http://www.cmmp.ucl.ac.uk/\~{}mjg
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 3210 Modeling
DE: 3230 Numerical solutions
DE: 1015 Composition of the core
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting