One of the principal means of understanding upper mantle dynamics involves inferring mantle flow directions from seismic anisotropy under the assumption that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment of a axes and (ii) when melt segregates and forms networks of weak shear zones, strain partitions between weak and strong zones, resulting in an alignment of a axes 90 degrees from the shear direction in three-dimensional deformation. This orientation of a axes provides a new means of interpreting mantle flow from seismic anisotropy in partially molten deforming regions of Earth.

[1] We report the results of an investigation of the microstructural evolution of rocks composed of a silicate plus a molten metal sulfide phase deformed plastically in simple shear. Deformation experiments on samples of San Carlos olivine (Fo 90 ) + 3, 5, or 9 vol% iron sulfide reveal the segregation of iron sulfide melt from the solid polycrystalline olivine matrix into regions enriched in metallic melt separated by regions depleted in the metallic melt. Previously published experimental studies on core-composition metals in silicate rocks, performed under hydrostatic pressure conditions, suggested that metallic melts could not segregate from silicates by porous flow below a critical melt fraction of $0.05. In contrast, we demonstrate that metallic melts can segregate from solid silicates by grain boundary percolation assisted by deformation, down to a pinch-off melt fraction of $0.01. The segregated metallic melt can accumulate into bands containing melt fractions exceeding 0.3. Using a model network of melt-rich sheets with melt fractions of 0.3, calculated Earth-scale permeabilities are as high as 10 À9 m 2 . Segregation velocities for these sulfide melts can be up to 150 km/yr, fast enough for formation of Earth's core to proceed by deformation-induced segregation well within the geochemically determined timescale of 30 million years. Therefore plastic deformation can facilitate removal of large enough quantities of metallic liquid from a solid silicate mantle to satisfy the geochemical constraints of the inefficient core formation theory without invoking the presence of a magma ocean.

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Intriguing seismic observations have been made for the bottom 400 km of Earth's mantle (the D؆ region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) 3 postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of (Mg 0.91Fe0.09)SiO3-PPv under quasi-hydrostatic conditions up to the pressure (P)-temperature (T) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed (V ⌽) decreases by 2.4 ؎ 1.4% across the PPv transition. Combined with the predicted shear-wave velocity (V S) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the V ⌽ and VS anomalies at the D؆ region. Also, density increases by 1.6 ؎ 0.4% and Grü neisen parameter decreases by 21 ؎ 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies.equation of state ͉ mantle ͉ phase transition ͉ bulk sound speed ͉ Grü neisen parameter T he DЉ region is believed to play an important role for the dynamics of the mantle and the core. The recent discovery of the postperovskite (PPv) transition (1-3) at the P-T conditions relevant to the DЉ region has provided new opportunities to understand the seismic observations and dynamic processes in the region. First-principles calculations (1, 4, 5) have predicted drastic changes in some geophysically important properties across the PPv transition (6, 7). The unusual changes have been attributed to the fundamental differences in crystal structure between perovskite (Pv), a 3D network structure with cornersharing SiO 6 octahedra, and PPv, a 2D layered structure with both corner and edge sharing SiO 6 octahedra (1,4,8,9). Therefore, measurements of the crystal structure provide a fundamental test for the predicted properties of PPv. However, synthesis of an appropriate single crystal for PPv in its stability field is extremely challenging for current techniques, making Rietveld refinement the only plausible method for studying the crystal structure. Currently only a single Rietveld refinement (10) exists for MgSiO 3 -PPv at 116 GPa and 300 K (Table 1).Some theoretical studies (1, 4) have predicted that bulk sound speed (V ⌽ ) decreases across the PPv transition whereas shearwave velocity (V S ) increases. Shieh et al. (Table 2), suggesting a large increase in V ⌽ across the PPv transition. However, no pressure medium was used in this study. The larger amount of Fe in Mao et al. (12) may cause the difference, yet a first-principles calculation showed that Fe has little effect on the bulk modulus of PPv (13).We have measured x-ray diffraction patterns of (Mg 0.91 Fe 0.09 )SiO 3 -PPv under q...