RAPID GRAIN SIZE AND ORIENTATION MEASUREMENTS IN CARBONATES USING SEM AND EBSD

Electron backscatter diffraction (EBSD) is a scanning electron microscopic (SEM) tool that has been used successfully to visualize small-scale variability in grain and/or crystallographic orientation in geologic materials. To date, most EBSD studies in geology have been performed by structural geologists examining small-scale deformation structures. The aim of this study is to investigate the feasibility of using this tool for carbonate rocks. Specifically, EBSD is used to investigate grain size distributions and crystallographic orientations in a variety of carbonate fabrics.

Carbonate sediments preserve diverse fabrics that provide important information about depositional environment and diagenetic conditions. Grain or crystal size distributions as well as crystal morphology and orientation are critical petrographic variables in carbonates. In well-preserved limestones, where primary textures are retained, light microscopy is the principal tool for examining texture. In texturally altered carbonates, however, primary fabrics may be obscured. Furthermore, certain textures, such as herringbone cement, may require a universal stage to quantitatively evaluate.

SEM combined with EBSD can enhance petrographic data by providing an additional, quantitative means of examining carbonates. Its high resolution allows features within small grains to be examined. Furthermore, use of EBSD and the associated forescatter detector permit visualization of grain boundaries and changes in crystallographic orientation. Using EBSD, grain boundaries in limestone and dolostone can be easily delineated and grain size distributions can be calculated rapidly. Preferred crystallographic orientations, such as observed in fibrous cements and during recrystallization under directional stress, can also be quantified.

While SEM and EBSD are not a substitute for light microscopy, they provide a valuable source of high-resolution information about carbonates that is relatively easy to obtain. With environmental or variable-pressure SEM, sample coating is not required and the same polished sections can be used for light microscopy, electron microscopy, and microprobe analysis. Automated scanning makes evaluation of large regions possible, and software packages permit flexibility in data analysis.