Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H40: Focus Session: Earth and Planetary Materials I |
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Sponsoring Units: DMP DCOMP Chair: Robert Liebermann, COMPRES Room: Morial Convention Center 232 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H40.00001: Elasticity of (Mg,Fe)O through the spin transition of iron in the lower mantle Invited Speaker: Recently, the important question of spin-pairing transitions of iron from high-spin (HS) to low-spin (LS) states in ferropericlase (Ref. 1 and references therein) affecting the lower mantle's density and seismic-wave velocities has been recognized (2,3). Since knowledge of this deep and inaccessible region is derived largely from seismic data, it is essential to determine the influence of the spin transition on elastic wave velocities at lower-mantle pressures. Here we discuss the results of measurements of the elastic tensor of (Mg0.94Fe0.06)O up to 60 GPa using impulsive stimulated light scattering. We find that all tensor elements soften substantially through the HS to LS transition, and that the softening occurs over an extended pressure range from 40 GPa to at least 60 GPa at room temperature. By invoking a simple thermodynamic description (4) of the transition we can compare our results to literature compression data (2,5) obtained from material with the higher iron concentrations likely to be found in ferropericlase in the lower mantle. The agreement is good and thus suggests that the thermodynamic description is reasonable. This in turn allows us to predict the effect of high temperature on the transition; we find that as temperature is increased the transition region is extended (see also Ref. 6) and the magnitude of the softening decreases. We conclude that although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most if not all of the lower mantle. 1. J. Badro, et al., Science, 300, 789 (2003). 2. J. F. Lin, et al., Nature, 436, 377 (2005). 3. J. F. Lin, et al., Geophys. Res. Lett., 33, L22304 (2006). 4. T. Tsuchiya, R. M. Wentzcovitch, C. R. S. da Silva, S. de Gironcoli, Phys. Rev. Lett., 96, 198501 (2006). 5. Y. Fei, et al, Geophys. Res. Lett. 34, L17307 (2007). 6. J.F. Lin, et al., Science, 317, 1740 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H40.00002: Elasticity of ferropericlase at Earth's lower mantle conditions Renata Wentzcovitch, Joao Justo, Zhongqing Wu, Cesar da Silva The thermoelastic properties of ferropericlase Mg$_{1-x}$Fe$_{x}$O (x = 0.1875) across the iron high-to-low spin crossover at lower mantle conditions have been investigated by combining first principles calculations with a thermodynamics model of this system. At room temperature the transition is somewhat sharp and the effect on the bulk modulus is quite dramatic. Along a typical geotherm the transition should occur across most of the lower mantle with a noticeable bulk modulus reduction in the mid lower mantle. This transition should also alter noticeably the magnitude of velocity heterogeneities caused by lateral temperature changes. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H40.00003: Inter-site Partitioning of Iron in Wadsleyite at High Pressures Rose Perea, Boris Kiefer (Mg$_{1-x}$,Fe$_{x})_{2}$SiO$_{4}$ -- wadsleyite is thought to be the most abundant metal in the upper part of the Earth's transition zone (410 -520 km depth). Wadsleyite contains three crystallographically non-equivalent octahedral sites, M1, M2, and M3. Experimentally, it has been observed that the M2 site is depleted in iron relative to the M1 and M3 sites. This asymmetric partitioning may affect the melting temperature and the density of iron bearing wadsleyite and influence the transport of mass, momentum, and energy across the transition zone. We performed LDA and GGA first-principle calculations of ferrous iron substitutions in the three crystallographically distinct octahedral sites: M1, M2, and M3. At low pressures we find, as expected, that iron in wadsleyite adopts its high spin state. The enthalpy differences from our high-spin calculations are consistent with the experimental observations that the M2 site is depleted in iron and independent of the magnetic moment of iron. This finding indicates that the inter-octahedral site partitioning of iron is due to the presence of the d-orbitals. If this finding can be corroborated our results will affect the understanding of the partitioning of other divalent transition metals and geochemical trace elements in wadsleyite, the most abundant mineral in the upper part of the Earth's transition zone. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H40.00004: First principles studies on several major phase transitions in Earth upper mantle Yonggang Yu, Zhongqing Wu, Renata Wentzcovitch First principles quasi-harmonic free energy calculations have been conducted to study several important phase transitions in mantle minerals with compositions Mg2SiO4 and MgSiO3 under realistic Earth's mantle conditions. We find encouraging and similar level of agreement with experiments for thermodynamic and vibrational properties and phase transition boundaries in~all cases where comparisons between results and data are possible. We also find systematic trends: 1) equations of state and thermodynamic properties of single crystalline phases are best described by the LDA; 2) phase boundaries are bracketed by LDA and GGA results, with GGA offering the upper bound transition pressure and being somewhat closer to the experimental ones. A summary of these results is offered for forsterite, wadsleite, ringwoodite, periclase, perovskite, post-perovskite, low clino-, and high clino-enstatite, and garnet, as well as~phase transitions between them. In overall these results can supplement high-PT experimental data on these minerals. (Research supported by NSF/EAR 013533, 0230319, 0635990, and NSF/ITR 0428774 (VLab), and Minnesota Supercomputing Institute.) [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H40.00005: The Phase Diagram of Portlandite, Ca(OH)$_{2}$ at Pressures up to 40 GPa Boris Kiefer, Megan Lockwood Hydroxides have attracted significant scientific interest over the past decades. They have been used as analogues to further our understanding of hydrogen bonding in complex materials. Several experimental observations suggest that portlandite undergoes reversible solid state amorphization at high pressures but the cause for this transition and its appearance in portlandite remains unknown. We performed static (0 K) first-principle calculations based on the GGA within the framework of Density-Functional-Theory in order to explore the phase diagram of portlandite for pressures up to 40 GPa. All structures were fully relaxed to determine the ground state of portlandite. We find that a split-site model for the hydrogen array is energetically preferred consistent with neutron diffraction experiments. At a pressure of $\sim $4.5 GPa we observe a phase transition from trigonal to monoclinic symmetry in agreement with previous studies. At all higher pressures we find that the monoclinic structure is the ground state of portlandite. However, we identified several energetically comparable structures. This indicates that the potential energy surface of portlandite has a surprisingly complex structure. The interplay of these structures suggest that the solid state amorphization is driven by non-hydrostatic stress and allows to rationalize experimentally observed differences between portlandite powders and single-crystals. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H40.00006: High PT elasticity within the quasiharmonic approximation with relaxed thermal stresses Pierre Carrier, Joao F. Justo, Renata M. Wentzcovitch We describe in detail a method to compute high PT elasticity within the quasiharmonic approximation (QHA). This approach differs from the usual formulation used to compute the statically constrained high PT elastic constants by including corrections due to deviatoric thermal stresses. The formulation is general and valid for crystals with up to triclinic symmetry. We use perovskite and post-perovskite phases of MgSiO3 to exemplify the use of the method to calculate elasticity and crystal structures at high PT. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H40.00007: Quantum Monte Carlo Study of Elastic Instability in Stishovite K.P. Driver, R.E. Cohen, P.L. Rios, M.D. Towler, R.J. Needs, J.W. Wilkins Stishovite is a octahedrally coordinated polymorph of silica which is stable at pressures within Earth's lower mantle (10 GPa). Elastic properties of stishovite are important for explaining seismic structure and it serves as a model system for other six-coordinated silicates. Near 50 GPa, stishovite transforms to the $\rm CaCl_{2}$-type structure due to an instability in the elastic shear modulus. The instability was first predicted by density functional theory (DFT) calculations and later confirmed by Raman spectroscopy and X-ray diffraction. Quantum Monte Carlo accurately predicts elastic constants and benchmarks previous DFT results on the stishovite elastic instability. Over the pressure range of 0 to 50 GPa, QMC shows the elastic shear modulus softens from 270 to 0 GPa in agreement with previous DFT and experimental results. Computations were performed at NERSC. Funding provided by the NSF (EAR-0530282, EAR-0310139) and the DOE (DE-FG02-99ER45795). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H40.00008: Pressure dependence of lattice anharmonicity and phonon lifetime in MgO: a first-principles calculation and implications for lattice thermal conductivity Xiaoli Tang, Jianjun Dong We report a recent first principles calculation of harmonic and anharmonic lattice dynamics of MgO. The 2$^{nd}$ order harmonic and 3$^{rd}$ order anharmonic interatomic interaction terms are computed explicitly, and their pressure dependences are discussed. The phonon mode Gr\"{u}neisen parameters derived based on our calculated 3$^{rd}$ lattice anharmonicity are in good agreement with those estimated using the finite difference method. The phonon lifetime due to lattice anharmonicity is calculated based on the single mode excitation approximation (SMEA). We have further estimated the isotope effect on phonon lifetime within the random mass disorder approximation. The implications for lattice thermal conductivity at high pressure are discussed based on a simple kinetic transport theory. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H40.00009: Pure Iron Compressed and Heated to Extreme Conditions Arkady Mikhaylushkin, Sergei Simak, Leonid Dubrovinsky, Natalia Dubrovinskaia, Borje Johansson, Igor Abrikosov The results of a first-principles study supported by the temperature-quenched laser-heated diamond anvil-cell experiments on the high-pressure high-temperature structural behavior of pure iron are reported. We show that in contrast to the widely accepted picture, the face-centered cubic (fcc) phase becomes as stable as the hexagonal-close-packed (hcp) phase at pressures around 300–360 GPa and temperatures around 5000–6000 K. Our temperature-quenched experiments indicate that the fcc phase of iron can exist in the pressure-temperature region above 160 GPa and 3700 K, respectively. This, in particular, means that the actual structure of the Earth's core may be a complex phase with a large number of stacking faults. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H40.00010: First-principles study of MgSiO$_{3}$ at core-mantle boundary conditions Siu-Chung Sung, Jones Tsz-Kai Wan Perovskite MgSiO$_{3}$ is an important mineral in geoscience studies. It plays a crucial role in the understanding of geophysical and geochemical activities taken place in the Earth's interior. In this talk, we report our recent work on First-principles molecular dynamics (FPMD) simulations of solid MgSiO$_{3}$ perovskite and post-perovskite, and molten MgSiO$_{3}$ at core-mantle boundary (CMB) conditions. The equations of state are determined at pressures up to 200 GPa and temperatures up to 6000K. The post-perovskite phase is found to be favoured over the perovskite at pressures above 102 GPa at zero temperature. Melting of MgSiO$_{3}$ has been observed by heating both perovskite and post-perovskite at high temperatures ($\sim$6000 K). The melting curve and electronic structures of solid and molten MgSiO$_{3}$ are also presented. Our simulated results thus provide useful constraints on structure and phase stability of MgSiO$_{3}$, which is the key to the understanding of deep-earth phenomena, such as the D$''$ discontinuity and seismic anisotropies in D$''$ layer. More importantly, the phase transformation of MgSiO$_{3}$ studied in this work provides insights into other aspects of geosciences like chemical heterogeneity and mantle convection, which may lead to a better model of the Earth's evolution. [Preview Abstract] |
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