A New Approach to Color Cathodoluminescence Imaging of Mudstones in the Scanning Electron Microscope (SEM)

Paul Mainwaring¹ and Juergen Schieber^2
1 GATAN Inc, Pleasanton, CA
² Indiana University, Bloomington, IN

Cathodoluminescence (CL), the light emission of a sample that is bombarded with electrons, can be observed with a petrographic microscope (floodgun) or with a SEM where other detectors aid mineral identification. Increasing interest in live color SEM-CL imaging of sedimentary rocks by the oil and gas industry spurred development of a new live color CL detector. Previously, SEM-CL color images required color mixing of filtered red, green and blue monochromatic images in a graphics program. The new detector, in contrast, permits live digital color imaging at resolutions of up to 4K X 4K. Petrographic characteristics can be observed immediately and chemical information can be gathered via X-Y coordinates on the color image. SEM operating conditions, required to improve the visibility of features such as multiple generations of overgrowths, can be adjusted in real time rather than waiting until a set of individual R, G, and B colored CL images has been mixed off-line. Immediate recognition of important petrographic characteristics greatly improves sample throughput.

In mudstones, the new detector allows provenance determination of quartz silt grains as small as a few microns in size. Just like with traditional sandstone CL, provenance is determined from the prevalent CL color, aided by grain-internal CL textures where available. Instant estimates of the relative importance of different source rocks (plutonic, metamorphic, volcanic) are possible, as well as identification of recycled sedimentary material, and quartz grains that formed during early diagenesis from opaline silica tests. CL spectroscopy of authigenic quartz grains will also be presented.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005