HR: 1340h
AN: MR23B-0066    [Abstracts]
TI: The Effect of Aluminum on the Elasticity of Post-Perovskite at High Temperatures
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
AB: One of the unsolved problems in geophysics and mineral physics is the development of a consistent model of the Earth's interior. Seismological observations are invaluable in this effort and contain, in principle, a wealth of information on the Earth's composition, as well as its thermal and theological structure. However, interference effects of phase, composition, pressure, and temperature complicate the interpretation of these observations. Mineral physics assumes a key role in determining the contribution of each of these factors to the observations. On the basis of experimental and theoretical mineral physics, it is thought that the chemistry of the Earth's lower mantle is dominated by phases with compositions close to MgSiO₃. Compositional models of the Earth's lower mantle commonly show that this phase contains ~10 mol% iron and ~4-5 mol% aluminum. The proximity of the expected composition to endmember MgSiO₃ perovskite has spawned large experimental and theoretical efforts to explore the elastic properties of this phase. However, the effect of minor elements on elastic properties, particularly at high temperatures, remains unknown. Ab-initio molecular dynamics simulations allow exploration of the effects of minor elements such as aluminum on elastic properties of post-perovskite. Simulations for aluminum bearing post-perovskite were performed in the pressure range of 90 - 150 GPa and 2000 - 4000 K, a parameter range that likely encompasses the Earth's lowermost mantle. Ultrasoft pseudopotentials in conjunction with the LDA were used to describe electronic interactions. The temperature was controlled by a Nose thermostat and the atomic dynamics were followed with femtosecond resolution for at least 2000 time steps. Simulations were performed for 80 atom supercells with an aluminum content of 6.25 mol% (two aluminum atoms). Following the stress evolution of unstrained and strained supercells provides the necessary information to obtain the equilibrium pressure as well as the complete elastic constant tensor at high pressures and temperatures. Preliminary results show that adding 6.25 mol% aluminum to MgSiO₃ post-perovskite reduces the aggregate bulk and shear moduli at 137 GPa and 2000 K by 0.6% and 2.6%, respectively. This trend, in which the bulk modulus is less affected than the shear modulus, is consistent with previous static calculations (0 K). However, the reduction in the bulk and shear modulus is only ~50% and ~35% of their static values. Thus the effect of aluminum on elastic properties may decrease with increasing temperature and revert at temperatures between 2500 - 3000 K. Therefore, higher mantle temperatures may be needed in the presence of aluminum to match seismic observations.
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005