APPLICATION OF SCANNING-CATHODOLUMINESCENCE IMAGING OF QUARTZ TO PROVENANCE ANALYSIS

Quartz is the most abundant constituent of most sandstones. Because it can be derived from a variety of primary (volcanic, plutonic, metamorphic) rocks, effective provenance analysis requires application of special techniques to link quartz to its source rocks. Previous techniques have included: (1) characterization of undulatory extinction and polycrystallinity (laborious, controversial), (2) trace-element analysis (requires use of an ion probe or other specialized equipment), (3) Fourier shape analysis (complicated by sediment transport and diagenesis), and (4) observation of cathodoluminescence color with a luminoscope (subjective; problems with resolution and color overlap).

The recent development of cathodoluminescence (CL) detectors that can be attached to a scanning electron microscope (SEM) now makes possible acquisition of high-resolution, high-magnification grayscale CL images. Quartz from volcanic, plutonic, and metamorphic rocks displays distinctively different textures or fabrics (e.g. growth zones, healed fractures) in SEM-CL images that make possible effective discrimination among quartz from different sources. Scanning-cathodoluminescence imaging allows rapid, reliable identification of volcanic, plutonic, metamorphic, and hydrothermal quartz in standard, polished thin sections. Once acquired, original CL fabrics are apparently unaffected by weathering, sediment transport, diagenesis, or geologic age. We have observed characteristic CL fabrics in quartz from primary rocks ranging in age from Holocene to Precambrian, in highly weathered soil profiles, and in sandstones from burial depths exceeding 4000 m. Original CL fabrics are destroyed and reset by metamorphic recrystallization at temperatures above ~300-400o C. The SEM-CL method is most effective with quartz grains larger than ~50 microns.