| Growing concern regarding the accelerated rate of carbon input to the atmosphere has prompted the search for comparable events throughout Earth's history. Recent research has attempted to determine the cause of such events, represented as delta13C excursions within the sedimentary record, and their subsequent effect on the Earth. It has been suggested by other researchers that such excursions could be the result of the large-scale release of 13C-depleted thermogenic methane produced by the intrusion of large igneous provinces (LIP) into organic-rich rocks and/or coal (e.g., Svenson et al., 2004, 2007; McElwain et al., 2005). Our study asserts that if these models are to be applied to the large-scale release of 13C-depleted thermogenic methane from intruded coals, 13C-enriched coal should he observed adjacent to the intrusions.;Using examples from coals intruded by both sills and dikes from several basins (Illinois, Karoo, and Piceance basins), we emphasize the importance of maceral content and rank at time of intrusion on the ultimate geochemical, petrographic, and isotopic composition of the heat-altered coals. Most of our examples show significant bulk geochemical and petrographic changes within the alteration haloes; for example, volatile matter decreases towards the intrusion while fixed carbon and vitrinite reflectance both increase. Intruded coals may show development of high reflectance, mosaic structures, natural coke, devolatilization vacuoles, carbonate mineralization, and pyrolytic carbon formation. However, no transects show any significant 13C org enrichment of the coal as the intrusion is approached, with shifts only on the order of 0.4 to slightly more than 1‰, hardly what might be expected if the intrusion had resulted in a significant release of 13C-depleted gases. Some isotopic shifts (e.g., in the Karoo Basin) appear to reflect changes in petrographic composition more than any other factor. This study underscores the importance of the use of organic petrography in combination with isotope geochemistry to sufficiently determine if contact metamorphosed coals could have released a significant amount of 13 Corg-depleted volatiles leading to negative delta 13C excursions in the sedimentary record. |
