Optical properties of organic matter in relation to thermal gradients and structural deformation

Abstract

The general response of macerals to the coalification process, the effectiveness of organic geochemical and petrological parameters as estimators of coalification and the applicability of different microscopical parameters in the assessment of rank are reviewed. The central role of vitrinite reflectance as the most widely used estimator of coalification from brown coals to the low-grade metamorphic zone is established on the basis of its relatively uniform variation with rank, specificity, ease and rapidity of measurement and cheapness. Factors influencing the optical properties of vitrinites are considered. Temperature has the dominant influence in modifying optical properties with rank, but time, static pressure and stress can all have significant influences upon the course of optical properties, depending upon particular geological conditions. Static pressure has a retarding effect on the coalification process, but it is also responsible for creating increasing anisotropy within the condensing aromatic structures as rank rises. Hydrostatic pressures and pore pressures can contribute to the overpressuring of sediments, thus affecting their thermal conductivities and consequently the rate of coalification. Stress is responsible for the development of biaxicity in the reflectance indicatrix. Fast and high levels of heating from igneous bodies cause textural effects on vitrinites as a result of carbonization that are quite different from those of coalification. Recognizing these different influences upon the optical properties of vitrinites, examples are given of the use and application of vitrinite reflectance in the preparation of rank maps, in the burial histories of basins, in the timing of coalification in relation to orogenesis and igneous activity and in the zones of anchimetamorphism and low-grade metamorphism. Consideration is also given to the problem of `provincialism' in correlations between chemical and physical parameters and to the future development of optical studies in organic petrology and their relation to organic geochemical investigations.