Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Geological Society of America Annual Meeting & Exposition
Cathodoluminescence Imaging in Sandstone Provenance Analysis: Quartz and
Feldspar Grains, Zircons, and Siliceous Rock Fragments
Robert M. Reed and Kitty L. Milliken
The light-magnifying qualities and improved resolution of photomultiplier-aided, SEM-based cathodoluminescence systems (scanned-CL) provide detailed images of grain textures that are useful in provenance analysis of sandstones. Scanned CL imaging can be further enhanced by using color filters to produce synthetic color images or limited wavelength images. These images commonly reveal textures that are invisible or obscure in panchromatic gray-scale CL images. Color scanned CL of quartz grains is particularly useful in identifying internal or external authigenic quartz cement that predates deposition.
Examination of over 100 siliciclastic units has shown that CL textures in quartz grains are not only useful for identifying the lithological provenance of some grains, but also the predepositional deformational history of some grains. This deformational history can provide the structural provenance of the grain. For example, grains that have undergone significant brittle deformation prior to deposition are typically distinguishable using scanned CL. If scanned CL images are not acquired, broken and recemented quartz grains can be mistaken for ductily deformed grains with discontinuous undulatory extinction.
Scanned CL imaging of feldspar and zircon grains and siliceous rock fragments can also aid in sandstone provenance analysis. Zircon grains typically show intense and complicated zoning in scanned CL images. Potential exists for differentiating between plutonic, volcanic, and metamorphic zircons based on this CL zoning, although there is some ambiguity and further study is needed. Both individual zircon grains and zircons contained within other types of grains provide opportunities for interpretation. Siliceous rock fragments commonly show complex CL textures. In CL images, some chert grains show distinct evidence of biologic components, while others show layered structures that suggest chemical precipitation. Scanned CL imaging has the potential to enable significant advances in the discrimination between microcrystalline quartz of sedimentary origin and that related to alteration of volcanic rocks. Feldspar grains do not show the range of CL textures seen in quartz grains, but textures reflecting the origin or predepositional alteration of the grains can be observed.