FABRIC CUT EFFECT: AN EBSD CAUTIONARY TALE

The investigation of igneous rocks using electron backscatter diffraction (EBSD) is gaining popularity. The method of EBSD identifies both phase and phase orientation, thus allowing for automated grain boundary detection and the characterization of fabric. As EBSD becomes a standard tool in the igneous petrologist’s toolbox, it is important to be aware of the potential methodological pitfalls. This study addresses a fabric artifact arising from the analysis of inequant crystals in thin-section. The motivation for this study were the results of EBSD analyses from the upper portion of a doleritic sill that repeatedly produced similar crystallographic preferred orientations, despite being collected without sample orientation preserved, suggesting the presence of a methodological artifact. To test this, crystal populations were synthetically generated using a numerical model. These crystal populations have set morphology and were randomly oriented and placed within a volume. An intersection plane (i.e., the “thin-section”) sampled crystals from the volume. While crystal orientations were uniformly distributed within the volume, the crystals sampled by the intersection plane appear to be oriented in a weak lineation. The strength of this fabric scales with the anisotropy of crystal morphology, with equant crystals producing no fabric artifact. This fabric artifact is similar to the well-known “cut effect” artifact that occurs when measuring crystal sizes in thin-section, where larger grains are more likely to be sampled than smaller grains. However, in the case presented here, there is a disproportionately large sampling of crystals with long-axis oriented normal to the intersection plane. The resulting fabric artifact appears as a lineation along this crystallographic axis perpendicular to the thin-section. In samples that do not have a fabric, this weak lineation acts as a test for the crystallographic long-axis. In general, these results suggest a minimum fabric strength cutoff, dependent on the morphology of the grains in question. If the fabric strength is near this cutoff, the distribution of crystal orientations should be interpreted as being uniform. This is of particular importance to the igneous petrologist, because igneous rocks commonly have relatively weak fabric.