From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, November 02, 2007
Color SEM-CL as a quartz grain provenance indicator
Fracture Research and Application Consortium, Bureau of Economic Geology
Abstract: (John Hooker, Steve Laubach)
A study of scanning electron microscope-based cathodoluminescence (SEM-CL) signatures from 530 quartz grains from gneisses, sandstones, igneous rocks, and natural sand samples is presented. SEM-CL textures (such as the presence of microfractures, zoning, mottling, etc.) can be combined with colors and linked to the type of source rock, and therefore be used as a provenance tool for sandstones in which rock fragments and other minerals are in low abundance.
Color is measured from images taken through red, green, and blue filters; each quartz grain is then characterized as red, purple, blue, or dark. Textureless grains are interpreted as low-grade- or contact-metamorphic if red and igneous if blue. Dark grains are interpreted as recycled authigenic quartz. Gneissic grains are invariably mottled and purple with sealed microfractures whose relative brightness varies with metamorphic grade. Blue grains show the most variation in texture; they may feature mottling, various types of microfractures, and CL-dark spots of consistent size (~5 microns) and density. Examination of samples from the Lewisian Gneiss and the Torridonian Supergroup from northwest Scotland supports previous studies that showed the Torridonian Supergroup's source rock was primarily gneissic initially, but became more varied over time. The method reveals that a sand sample from the shore of Loch Tarbhajdh was sourced primarily from the Torridon Group and not the Lewisian Gneiss. The method also reveals a varied source for the sediment of the La Boca Formation (Triassic-Jurassic), and a metamorphic source for the quartz grains of the Lower Wilcox Group (Paleocene). Quartz grains from Galveston Island sediment have a substantial recycled sediment component.