OBSERVING AND QUANTIFYING DEFORMATION IN MINERALS BY IN SITU 2D X-RAY DIFFRACTION

Minerals are valuable recorders of deformation in rocks, within igneous, metamorphic, crustal and mantle settings on Earth or other planetary bodies. Non-uniform stress, such as shear stress during tectonic processes, or shock metamorphism from meteorite impact, produces non-uniform strain or ‘bending’ and disruption of mineral grains.

In situ two-dimensional X-ray diffraction (2D XRD) is a powerful technique that reveals strain-related modification of crystal structure at sub-mm scale as well as textural relations between minerals. In situ XRD works in tandem with polarizing microscopy; minerals displaying undulatory extinction optically, exhibit streaking by 2D XRD, whereas minerals displaying mosaicism (such as shocked quartz), or chessboard extinction, optically, exhibit asterism by 2D XRD. While the polarizing microscope gives a qualitative assessment of strain, in situ XRD on the same mineral grain provides a complementary quantitative assessment of strain, and can extend our observations to whole rocks or rock slabs, not amenable to microscopy. Different minerals measured within the same rock can act as independent recorders of the deformation event.

2D XRD data were collected using a Bruker D8 Discover µXRD with CuKα or CoKα radiation, a parallel beam optics system with a beam diameter of 300 or 500 µm. 2D XRD data were collected using an area detector. The 2D XRD image reveals the crystallinity and texture of the minerals under the X-ray beam. Polycrystalline materials diffract X-rays in all directions, producing continuous Debye rings in the 2D image. Unstrained single crystal targets scatter X-rays in a single direction, producing individual X-ray spots in the 2D image. Non-uniformly strained crystals contain a mosaic spread of subgrain orientations within a single mineral grain (called strain-related mosaicity - SRM), producing streaks along the Debye rings or chi direction (χ) in the 2D image. Streaking is quantified by measuring its full width at half maximum along χ (FWHM_χ). When the mineral subgrains or subdomains are large (>15 µm), the X-ray streak is resolved into a row of diffraction spots, called asterism. Asterism can be quantified by summing the peak widths for each spot in the row (ƩFWHM_χ), enabling reconstruction of the SRM in the original strained grain. This texture is pervasive in quartz and common in feldspar and olivine, but not pyroxene.

Deformation in minerals is often accompanied by twinning, slip system formation and recrystallization; these effects are not straightforward to decipher by in situ XRD.