Use of Cathodoluminescence Petrography for Understanding Compaction, Quartz Cementation, and Porosity in Sandstones
Published:January 01, 1991
David W. Houseknecht, 1991. "Use of Cathodoluminescence Petrography for Understanding Compaction, Quartz Cementation, and Porosity in Sandstones", Luminescence Microscopy and Spectroscopy: Qualitative and Quantitative Applications, Charles E. Barker, Robert C. Burruss, Otto C. Kopp, Hans G. Machel, Donald J. Marshall, Paul Wright, Helen Y. Colbum
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Cathodoluminescence (CL) petrography provides a unique capability for documenting physical and chemical diagenetic processes in sandstones, particularly those cemented by quartz. Mostdetrital quartz grains display relatively intense CL owing to lattice defects and trace cation inclusions inherited from crystalline source rocks. In contrast, most authigenic quartz contains fewer lattice defects and trace cations, and therefore displays less intense CL.
Detrital grains, cement, and porosity can be objectively identified and measured using CL petrography. These measurements allow quantitative evaluation of total compaction, which is inferred by calculating intergranular volume (intergranular cement plus intergranular porosity), and cementation. An estimate of chemical compaction can also be made by subjectively reconstructing original grain boundaries at intergranular pressure solution contacts and measuring overlap quartz. These quantitative estimates can be used to interpret silica budgets, to evaluate porosity, and to assess the relative importance of various diagenetic events to porosity reduction.
CL petrography also permits valuable qualitative observations that can rarely be made using only transmitted light techniques. Brittle grain fracture and plastic deformation of ductile lithic fragments, both indicative of mechanical compaction, can commonly be documented in CL. Cement zonation, particularly in quartz overgrowths, can provide the key to unraveling complex paragenetic relationships.
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Luminescence Microscopy and Spectroscopy: Qualitative and Quantitative Applications
The papers presented in this volume make it clear that luminescence microscopy and spectroscopy are being employed in an ever wider array of geological studies. The editors suggest several ways that luminescence studies can be employed or improved: (1) to assist in the integration of trace element, isotope, fluid inclusion and mineral studies using CL results to assure that the same zones and (or) mineral compositions are utilized; (2) more reliable tracing of zones whether microscopic or of regional extent; (3) better interpretation of diagenetic, mineralization and alteration events because of the control and discrimination of crosscutting relationships and subtle changes in chemistry that often become obvious using luminescence; (4) as a tool for direct detection of rare earth element deposits, Mississippi Valley type Pb-Zn ores and in some cases oil reservoirs; (5) introduction of standard materials and methods for calibrating spectrometers and possibly increasing the uniformity of subjective observations; (6) improvements in instrumentation to diminish thermal quenching effects at the same time gains are necessary in the level of activation of luminescence and in the quality of the microscopic image transmitted to the observer; (7) more efforts at experimental determination of the causes of luminescence and their interpretation relative to conditions that exist in natural systems.