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The seismic anisotropy of the Earth’s solid inner core has been the topic of much research. It could be explained by the crystallographic preferred orientation (CPO) developing during convection. The likely phase is hexagonal close-packed iron (hcp), alloyed with nickel and some lighter elements. Here we use high energy synchrotron X-rays to study CPO in Fe-9wt%Si, uniaxially compressed in a diamond anvil cell in radial geometry. The experiments reveal that strong preferred orientation forms in the low-pressure body-centred cubic (bcc) phase that appears to be softer than pure iron. CPO is attributed to dominant {110}<111>slip. The onset of the bcc→hcp transition occurs at a pressure of ≈15 GPa, and the alloy remains in a two phase bcc+hcp state up to 40 GPa. The hcp phase forms first with a distinct {11¯20} maximum perpendicular to compression. Modelling shows that this is a transformation texture, which can be described by Burgers orientation relationship with variant selection. Experimental results suggest that bcc grains oriented with <100> parallel to compression transform into hcp first. The CPO of the hcp changes only slowly during further pressure and deviatoric stress increase at ambient temperature. After heating to 1600 K, a change in the hcp CPO is observed with alignment of (0001) planes perpendicular to compression that can be interpreted as dominant (0001)<11¯20> slip, combined with {10¯12}<¯1011> mechanical twinning, which is similar to the deformation modes suggested previously for pure hcp iron at inner core conditions. 

Previously, synchrotron X-ray Laue microdiffraction has been used to measure the magnitudes of residual strain in materials. Recently the method was advanced to determine the orientation of the strain ellipsoid and applied to naturally deformed quartzites; however, the deformation history of these quartzites is ambiguous due to their natural origin. In this study, synchrotron X-ray Laue microdiffraction (µXRD) is used to measure the residual strain for the first time in a sample with known stress history, rolled titanium. A deviatoric strain tensor is calculated from each Laue diffraction image collected with two µXRD scans of a rolled titanium sheet in different sample orientations. The principal strain axes are calculated using an eigen decomposition of the deviatoric strain tensors. The results show that the principal axis of compression is aligned with the normal direction of the titanium sheet, and the principal axis of extension is aligned with the rolling direction. Pole figures are used to represent the 3D distribution of residual strain axes. 

The introduction of multigrain crystallography (MGC) applied in a laser-heated diamond anvil cell (LH-DAC) using synchrotron X-rays has provided a new path to investigate the microstructural evolution of materials at extreme conditions, allowing for simultaneous investigations of phase identification, strain state determination, and orientation relations across phase transitions in a single experiment. Here, we applied this method to a sample of San Carlos olivine beginning at ambient conditions and through the α-olivine → γ-ringwoodite phase transition. At ambient temperatures, by measuring the evolution of individual Bragg reflections, olivine shows profuse angular streaking consistent with the onset of yielding at a measured stress of ~1.5 GPa, considerably lower than previously reported, which may have implications for mantle evolution. Furthermore, γ-ringwoodite phase was found to nucleate as micron to sub-micron grains imbedded with small amounts of a secondary phase at 15 GPa and 1000 °C. Using MGC, we were able to extract and refine individual crystallites of the secondary unknown phase where it was found to have a structure consistent with the ε-phase previously described in chondritic meteorites. 

Understanding dynamics across phase transformations and the spatial distribution of minerals in the lower mantle is crucial for a comprehensive model of the evolution of the Earth’s interior. Using the multigrain crystallography technique (MGC) with synchrotron x-rays at pressures of 30 GPa in a laser-heated diamond anvil cell to study the formation of bridgmanite [(Mg,Fe)SiO 3 ] and ferropericlase [(Mg,Fe)O], we report an interconnected network of a smaller grained ferropericlase, a configuration that has been implicated in slab stagnation and plume deflection in the upper part of the lower mantle. Furthermore, we isolated individual crystal orientations with grain-scale resolution, provide estimates on stress evolutions on the grain scale, and report {110} twinning in an iron-depleted bridgmanite, a mechanism that appears to aid stress relaxation during grain growth and likely contributes to the lack of any appreciable seismic anisotropy in the upper portion of the lower mantle. 

In recent years there have been several attempts to make the link between mineral properties and seismic anisotropy in the D’’ region but have yet to reach consensus with regards to the dynamics in lower mantle minerals that could give rise to the observed seismic anisotropy. Here, we aim to provide further constraints on the observed long wavelength shear velocity patterns seen in seismic tomography studies. We introduce a forward model of deformation in a subducting slab as it impacts the core mantle boundary (D’’ layer) and proceeds to upwelling at the edge of a simulated LLSVP. By implementing the most recent results from atomistic modeling and high-pressure deformation experiments coupled with a 3-dimensional geodynamic model, we compare the microstructural evolution of an aggregate with a pyrolytic composition to the macroscopically observed seismic anisotropy of the lowermost mantle. We account for topotaxial relations in the forward and reverse phase transitions of MgSiO3-perovskite (Pv) to post-perovskite (pPv) within the slab as well as explore the effects introduced by partial melting near the CMB. Comparisons in the two leading candidate deformation mechanisms in the post-perovskite phase, (001) and (010), are compared. In this study we find that the reverse transition (pPv to Pv) occurs at a depth which is ~ 150 km deeper than that of the forward transition due to increasing temperature near the CMB providing a varying topography of the D’’ discontinuity. Our model also produces good fits with the isotropic velocities of PREM for the bulk lower mantle. When coupled with temperature and pressure dependent forward and reverse phase transitions, a pPv system with dominant (001) slip provides good correlation with the currently observed VSH fast horizontal (~ 1 – 6%) in D’’ and with VSV consistently fast in upwelling areas. Azimuthal variations along the streamline are also investigated showing a symmetry lower than that of the assumed VTI in D’’ introduced by ‘rolling’ effects near the slab’s edge. The addition of 1% partial melting at the CMB is shown to increase S and P wave anisotropy beneath the slab at the base of upwelling with up to ~2.5 & 4.0% P and S wave reductions respectively compared to the global reference.