Melt segregation and strain partitioning: implications for seismic anisotropy and mantle flow

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.

Quantitative texture analysis of small domains with synchrotron radiation X-rays

Quantitative analysis of crystallographic preferred orientation (texture) of very small volumes in fine-grained polycrystalline materials has been carried out with a monochromatic X-ray microbeam (≤30 µm) at the microfocus beamline of the European Synchrotron Radiation Facility (ESRF). The experimental procedure is described and illustrated with textures of rolled aluminium, aluminium and steel wires, polymer fibers and natural bone material (apatite).

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.

Aspects of X-ray diffraction on single spider fibers

Diffraction patterns of silk from several spider species have been obtained by synchrotron radiation using a beam size > or = 10 microm. Single fiber diffraction patterns were obtained for fiber diameters down to a few microns. Diffraction patterns recorded with a 10 microm wide X-ray beam displayed fiber texture. The presence of two fractions of different crystallinity was confirmed for a single Nephila clavipes fiber. The orientation distribution of the polymer chains of the crystalline fraction along the fiber axis was found to be about 23 degrees full-width at half maximum (fwhm). The azimuthal spread of the short-range order fraction was about 86 degrees fwhm.

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.