Cathodoluminescence can be an invaluable tool in petrographic studies. It provides information on the spatial distribution of trace elements in terrigenous clastic (as well as carbonate) grains and cements. Analysis can be done using polished rock chips, polished thin sections, or even unpolished and uncovered thin sections. The equipment requires costs about the same as a moderately priced polarizing microscope and can be installed on virtually any microscope.
This example shows a sandstone from the Devonian Hoing Sandstone Member of the Cedar Valley limestone in Illinois. The upper photo, taken with transmitted light, shows a quartz arenite with elongate, sutured intergranular boundaries which might be considered as indicative of compaction and pressure solution. The lower photo, taken with cathodoluminescence, shows the same field of view with dramatically different results. The detrital grain cores, which luminesce orange and blue, can be seen to be well rounded, and touch each other only at point contacts. Subsequent quartz overgrowths (generally nonluminescent with some luminescent zones) have obliterated most porosity and give the appearance of a compacted fabric when cathodoluminescence is not used. The differences in luminescence between detrital grain cores and authigenic overgrowths is a function of subtle differences in their trace element composition.
These differences are accentuated by long-exposure-time photography of small areas because of the inherent weakness of the luminescence. Photos by R. F. Sippel.