THE EFFECT OF HEATING RATE ON ORGANIC MATURATION: CONTACT METAMORPHISM OF COAL VERSUS BURIAL MATURATION

The importance of heating rate and different geothermal regimes on the thermal maturation of organic matter has been emphasized in the work of Murchison and co-workers (e.g., Murchison, 2004, 2006), and it has been suggested that coals that have been metamorphosed by igneous intrusion may follow a different geochemical track than those altered by normal coalification processes (e.g., Van Krevelen and Schuyer, 1957; Schapiro and Gray, 1966). Coals that have been exposed to higher geothermal gradients, and particularly those heated rapidly by intrusions, may show different reflectance-volatile matter relationships and other geochemical relationships when compared with those altered by burial metamorphism.

In an overview, data from several locales will be used to demonstrate the effect of igneous intrusions on the metamorphism of coals, including data from new sample suites from the Illinois, Raton, Piceance, and Karoo basins, and also from several extant data sets. These data typically show a decrease in volatile matter, hydrogen, oxygen, and nitrogen; and an increase in fixed carbon, ash, and carbon. Associated with these bulk geochemical changes, increases in vitrinite reflectance are observed along with the development of devolatilization vacuoles, natural coke, mosaic structures, small shifts in stable isotopic ratios, and the formation of pyrolytic carbon and neoformed minerals (carbonates). Subtle differences are observed between trends in these data and those for coals altered by normal burial maturation.

Comparing these data to extant geochemical data for coals that have undergone normal burial maturation, we will evaluate the hypothesis that if coal alteration occurs rapidly (due to igneous intrusion) rather than gradually (due to burial maturation), then different relationships are seen in chemical composition (proximate and ultimate analyses) and vitrinite reflectance.