Florian Heidelbach

Shear deformation experiments of forsterite at 11 Gpa - 1400°C in the multianvil apparatus

Synthetic forsterite samples were shear-deformed at 11 GPa, 1400°C in the multianvil apparatus. The deformation microstructures have been characterised by SEM, EBSD, X-ray diffraction peak broadening and strain anisotropy analysis, and TEM. Different time durations have been characterised with a view to follow the evolution of strain and stress in high-pressure deformation experiments. A high density of [001] dislocations is introduced during pressurization at room temperature although no significant macroscopic shear or crystal preferred orientations are induced at this stage. The deviatoric stress is probably on the order of 1.5 GPa. Heating at 1400°C leads to a rapid decrease of the density of these dislocations. The shear deformation at high-temperature leads to measurable strain and development of crystal preferred orientations after one hour. Stress and strain-rate continue to decrease with time, such that eight hour experiments exhibit microstructures where recovery is apparent. At this stage, the stress level is estimated at ca. 100 MPa from dislocation density measurements. Crystal preferred orientations and TEM characterisation show that glide of [001] dislocations on (100) or (010) is the dominant deformation mechanism. Further investigation is needed to determine whether inhibition of [100] glide in these experiments is due to the role of water or whether a physical effect of pressure is also contributing.

Crystal alignment of carbonated apatite in bone and calcified tendon: results from quantitative texture analysis

Calcified tissue contains collagen associated with minute crystallites of carbonated apatite. In this study, methods of quantitative X-ray texture analysis were used to determine the orientation distribution and texture strength of apatite in a calcified turkey tendon and in trabecular and cortical regions of osteonal bovine ankle bone (metacarpus). To resolve local heterogeneity, a 2 or 10 microm synchrotron microfocus X-ray beam (lambda = 0.78 A) was employed. Both samples revealed a strong texture. In the case of turkey tendon, 12 times more c axes of hexagonal apatite were parallel to the fibril axis than perpendicular, and a axes had rotational freedom about the c axis. In bovine bone, the orientation density of the c axes was three times higher parallel to the surface of collagen fibrils than perpendicular to it, and there was no preferential alignment with respect to the long axis of the bone (fiber texture). Whereas half of the apatite crystallites were strongly oriented, the remaining half had a random orientation distribution. The synchrotron X-ray texture results were consistent with previous analyses of mineral orientation in calcified tissues by conventional X-ray and neutron diffraction and electron microscopy, but gave, for the first time, a quantitative description.

Dislocation creep of magnesiowüstite (Mg0.8Fe0.2O)

Abstract Magnesiowustite is likely the second most abundant and probably weakest lower-mantle phase. Its deformation properties will therefore influence the rheological and dynamic behaviour of this region. In this study magnesiowustite aggregates with a composition of Mg0.8Fe0.2O have been deformed under dry conditions in axial compression at a confining pressure of 300 MPa and temperatures from 1200 to 1400 K (0.41

Crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep

Abstract The crystallographic preferred orientations of a series of experimentally deformed fine-grained albite aggregates were measured by synchrotron source X-ray diffraction. Most samples were deformed and extensively recrystallized by low-temperature recrystallization-accommodated dislocation creep. In axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on {001} ; the sheared samples have a marked asymmetry of the maxima with respect to the shear zone boundaries. One sample was axially compressed by solution precipitation creep; it developed a somewhat different but equally strong preferred orientation, perhaps reflecting crystallographic anisotropy in rates of dissolution and growth.

Texture analysis of a recrystallized quartzite using electron diffraction in the scanning electron microscope

Abstract The microstructure and crystallographic preferred orientation (texture) of a recrystallized quartzite from the Bergell Alps was quantitatively investigated using automated indexing of electron backscattered patterns (EBSP). The results are displayed in orientation maps of the microstructure and orientation as well as misorientation distribution functions. The orientation distribution function derived from EBSP compares excellently with texture data derived from independent neutron diffraction experiments. The misorientation distribution function of next neighbors reveals strong maxima for small angle grain boundaries (subgrain boundaries) and the Dauphine twin correlation. The Dauphine twins, which are generally not detectable optically, are not correlated with specific maxima in the orientation distribution function. Their origin and their role during recrystallization is discussed. During recrystallization no other preferred high angle misorientations developed. The frequent occurrence of small angle grain boundaries indicates progressive subgrain rotation as a recrystallization mechanism.

Superplasticity in garnet from eclogite facies shear zones in the Haram Gabbro, Haramsøya, Norway

Study of garnet adjacent to and within shear zones that underwent synkinematic eclogitization reveals microstructures and textures that, for the first time, indicate grain- boundary sliding as the main deformation mechanism in garnet. Monomineralic garnet layers exhibit characteristic features of superplastic behavior and show little strength contrast to the surrounding minerals. The low strength is caused by the synkinematic growth of garnet limiting the grain size to tens of micrometers. This behavior may significantly influence rheologic properties of high- and ultrahigh-pressure shear zones in subduction environments and may play a role during the formation of garnet in the transition zone of the mantle.

Compositional re-equilibration of garnet: the importance of sub-grain boundaries

Garnets from meta-granitoid high pressure rocks (Sesia Zone, Western Alps) show complex internal sub-grain textures in electron forescatter images. All investigated garnets consist of a large number of sub-grains with different shapes and sizes. Some garnets exhibit a sub-texture with very fine-grained (< 20 μm) sub-grains in their cores overgrown by palisade-like sub-grains in the rims. Sub-grain boundaries in these garnets have enabled diffusive element exchange between the garnet core and the surrounding matrix. Compositional mapping reveals zonation patterns of Mg that indicate modification of the garnet composition during prograde metamorphism. The extent of diffusional re-equilibration is dependent on sub-grain size and element diffusivities. Our samples show that XMg is strongly influenced by diffusion along the sub-grain boundaries, whereas apparently slow diffusing elements, such as Ca, Ti and Y preserve their original concentric zonation pattern. This differential re-equilibration leads to very complex chemical zonation that cannot be easily interpreted in terms of simple prograde growth zonation or of normally-applied spherical diffusion models. The observation that almost all garnets in the investigated samples exhibit a sub-grain pattern suggests this might be a common feature in high pressure/low temperature rocks.

Laue Orientation Imaging

Advantage was taken of the highly focused X-ray beam (10-30 microm) and the broad white spectrum of synchrotron X-rays at the ESRF for automatic recording of Laue patterns from polycrystals and extraction of orientation information. The procedure used is similar to that applied for electron-backscattering patterns in the scanning electron microscope and provides data for local orientation mapping used in texture analysis. Laue patterns are obtained from a thin slice of material in transmission and recorded with a CCD detector. The Laue geometry is converted into a gnomonic projection in which co-zonal reflections lie on straight lines. On applying the Hough transform these lines are merged into a single point, which is recognized by the computer and assigned zone indices [uvw] by comparison with a table of interzonal angles. From the angular positions of several [uvw] the crystal orientation is calculated. The method is illustrated for the orthorhombic magnesium silicate olivine.

The structure of a super-aluminous version of the dense hydrous-magnesium silicate phase D

Abstract The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi2H2O6, may be an important link in a chain of hydrous phases that carry H2O in the ultramafic portions of subducting lithosphere, into the Earth’s lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al2O3, using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg0.2Fe0.15Al1.8H1.8SiO6, was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60-70 μm long single crystals at similar conditions. The recovered crystals were slightly richer in H2O (Mg0.2Fe0.12Al1.5Si0.92H3.1O6) and their unit-cell parameters were similar to those of MgSi2H2O6 phase D. A refinement of the crystal structure was carried out in the P3̅1m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H2O within portions of subducted oceanic crust in the lower mantle

High-Temperature Deformation of Enstatite Aggregates †

Synthesized polycrystalline enstatite samples were deformed in a Paterson gas-medium apparatus at 1200–1300°C, oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 MPa (protoenstatite field) or 450 MPa (orthoenstatite field). At both confining pressures, the mechanical data display a progressive increase of the stress exponent from n = 1 to n~3 with increasing differential stress, suggesting a transition from diffusional to dislocation creep. Nonlinear least squares fits to the high-stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kJ/mol for orthoenstatite and protoenstatite, respectively. Deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy. Microstructures show undulatory extinction and kink bands, evidence of dislocation processes. Crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on (100)[001]. Most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae. While many lamellae may have formed during quenching from run conditions, those in samples deformed in the orthoenstatite field are often bordered by partial [001] dislocations, suggesting transformation due to glide of partial [001] dislocations in (100) planes. Comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions. However, as orthoenstatite has a higher activation energy and smaller stress exponent than olivine, this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle.