REVEALING THE SECRETS WITHIN IGNEOUS ROCKS: HIGH-QUALITY IMAGING FOR TEXTURAL ANALYSIS

Textural analysis can identify processes involved in the development of igneous rocks, from nucleation through to textural equilibration, revealing processes of magma recharge, mingling, and mixing in complex magmatic systems. There are many ways of gathering these data, e.g., 3D tomography, micro-XRF, QEMSCAN, etc. however these are expensive. Calculation of crystal size distributions are a cheaper way of obtaining textural data, but requires images of entire thin sections, where individual crystals and their edges can be identified. This project used an optical microscope with a motorized stage and two quarter wave plates to quickly obtain entire thin section photos, including in circularly polarized light (CPL). The quarter wave plates were used to view all minerals at their maximum birefringence (Higgins, 2010), no matter the orientation of the stage, helping distinguish crystal edges. An Evident (formerly Olympus) BM-53 microscope was used with the Stream Motion software to automatically adjust the focus and stitch together individual photographs to obtain a high-quality scaled photograph of an entire polished thin section in under 5 minutes, including setup of the area to be imaged.

This produced images, where even small plagioclase crystals within dike samples could be easily distinguished by the authors, three of whom had no prior experience with optical mineralogy. Plagioclase crystals as small as 0.1 mm in length could be confidently outlined using the Evident BM-53 photographs, with plane and circularly polarised light, when viewed within Adobe Illustrator. While there is a cost outlay for the purchase of a microscope with a motorized stage and appropriate image stitching software, it is considerably cheaper than many other instruments that can produce texture maps. Use of CPL, which only requires low cost quarter wave plates, allowed for easy identification of minerals and the edges of separate crystals. Hand digitization is a time-consuming process, especially compared to obtaining the images. However, with high-quality images, it is an easy to learn process for student researchers with minimal prior experience.

Higgins, M.D., 2010, Imaging birefringent minerals without extinction using circularly polarized light: The Canadian mineralogist, v. 48, p. 231–235, doi:10.3749/canmin.48.1.231.