[1] Room temperature investigations on the shear strength, elastic moduli, elastic anisotropy, and deformation mechanisms of MgO (periclase) are performed in situ up to pressures of 47 GPa using radial X-ray diffraction and the diamond anvil cell. The calculated elastic moduli are in agreement with previous Brillouin spectroscopy studies. The uniaxial stress component in the polycrystalline MgO sample is found to increase rapidly to 8.5(±1) GPa at a pressure of 10(±1) GPa in all experiments. Under axial compression, a strong cube texture develops which was recorded in situ. It is probable that the preferred orientation of MgO is due to deformation by slip. A comparison between the experimental textures and results from polycrystal plasticity suggest that the {110}h1 " 10i is the only significantly active slip system under very high confining pressure at room temperature. These data demonstrate the feasibility of analyzing elastic moduli, shear strength, and deformation mechanisms under pressures relevant for the Earth's lower mantle. Implications for the anisotropy and rheology of the lower mantle are discussed.

BEARTEX is a general PC-based Windows software package for quantitative texture analysis. The 30 programs that it contains provide corrections for experimental pole figures, orientation distribution calculations from complete or incomplete pole figures for all crystal and sample symmetries down to triclinic, graphical display of data, polycrystal tensor property determinations and various other operations.

Polycrystalline (Mg(0.9),Fe(0.1))SiO3 post-perovskite was plastically deformed in the diamond anvil cell between 145 and 157 gigapascals. The lattice-preferred orientations obtained in the sample suggest that slip on planes near (100) and (110) dominate plastic deformation under these conditions. Assuming similar behavior at lower mantle conditions, we simulated plastic strains and the contribution of post-perovskite to anisotropy in the D'' region at the Earth core-mantle boundary using numerical convection and viscoplastic polycrystal plasticity models. We find a significant depth dependence of the anisotropy that only develops near and beyond the turning point of a downwelling slab. Our calculated anisotropies are strongly dependent on the choice of elastic moduli and remain hard to reconcile with seismic observations.

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Polycrystalline MgGeO3 post-perovskite was plastically deformed in the diamond anvil cell between 104 and 130 gigapascals confining pressure and ambient temperature. In contrast with phenomenological considerations suggesting (010) as a slip plane, lattice planes near (100) became aligned perpendicular to the compression direction, suggesting that slip on (100) or (110) dominated plastic deformation. With the assumption that silicate post-perovskite behaves similarly at lower mantle conditions, a numerical model of seismic anisotropy in the D'' region implies a maximum contribution of post-perovskite to shear wave splitting of 3.7% with an oblique polarization.

Understanding deformation of mineral phases in the lowermost mantle is important for interpreting seismic anisotropy in Earth's interior. Recently, there has been considerable controversy regarding deformation-induced slip in MgSiO(3) post-perovskite. Here, we observe that (001) lattice planes are oriented at high angles to the compression direction immediately after transformation and before deformation. Upon compression from 148 gigapascals (GPa) to 185 GPa, this preferred orientation more than doubles in strength, implying slip on (001) lattice planes. This contrasts with a previous experiment that recorded preferred orientation likely generated during the phase transformation rather than deformation. If we use our results to model deformation and anisotropy development in the D'' region of the lower mantle, shear-wave splitting (characterized by fast horizontally polarized shear waves) is consistent with seismic observations.

Abstract. Experiments by Zhang and Karato [1995] have shown that in simple shear dislocation creep of olivine at low strains, an asymmetric texture develops with a [100] maximum rotated away from the shear direction against the sense of shear. At large strain where recrystallization is pervasive, the texture pattern is symmetrical, and [100] is parallel to the shear direction. The deformation texture can be adequately modeled with a viscoplastic self-consistent polycrystal plasticity theory. This model can be expanded to include recrystallization, treating the process as a balance of boundary migration (growth of relatively underformed grains at the expense of highly deformed grains) and nucleation (strain-free nuclei replacing highly deformed grains). If nucleation dominates over growth, the model predicts a change from the asymmetric to the symmetric texture as recrystallization proceeds and stabilization in the "easy slip" orientation for the dominant (010)[100] slip system. This result is in accordance with the experiments and suggests that the most highly deformed orientation components dominate the recrystallization texture. The empirical model will be useful to simulate more adequately the development of anisotropy in the mantle where olivine is largely recrystallized. IntroductionSimple shear experiments of rock-forming minerals and analogs deformed by dislocation creep reveal some puzzling features. After moderate strains, the deformed original grains develop a characteristically asymmetric preferred orientation pattern with a monoclinic symmetry, including a mirror plane perpendicular to the shear plane and containing the shear direction. These experimental simple shear deformation textures (in this paper the term texture is used synonymous with lattice preferred orientation), such as those for quartz [ These orthorhombic textures cannot be predicted with polycrystal plasticity theory for deformation by dislocation glide such as Taylor, Sachs, or self-consistent models. Authors have referred to the textures as "easy slip" orientations [e.g., Schmid and Casey, 1986], implying that a microscopic slip plane of an active slip system is parallel to the macroscopic shear plane and a microscopic slip direction is parallel to the shear direction, and therefore deformation on such a slip system in simple shear is "easy." So far, there has been no explanation to how crystals rotate and remain within those orientations. Herwegh and Handy [1996] suggest a "rigid body rotation" into the easy slip orientation and an unexplained cessation of rotation once those special orientations are reached.Experimental

The orientation distribution of a textured polycrystal has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by X-ray or neutron diffraction. A new method is demonstrated that uses the whole diffraction spectrum, rather than extracted peak intensities, by combining ODF calculation with Rietveld crystal structure refinement. With this method, which is illustrated for a synthetic calcite texture, it is possible to obtain quantitative texture information from highly incomplete pole figures and regions of the diffraction spectrum with many overlapping peaks. The approach promises to be advantageous for low-symmetry compounds and composites with complicated diffraction spectra. The method is particularly elegant for time-offlight neutron diffraction, saving beam time by using small pole-figure regions and many diffractions.