CONTACT METAMORPHOSED COAL AND GLOBAL WARMING: IS THERE EVIDENCE FOR A LARGE-SCALE RELEASE OF METHANE?

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 δ¹³C 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 igneous rocks into organic-rich rocks and/or coal (e.g., Svenson et al., 2004, 2007; McElwain et al., 2005). Although methane derived from coal is 25‰ lighter than the coal itself (Sackett, 1978), simple mass-balance calculations suggest that methane generated by such events would not significantly affect atmospheric carbon levels (even if all gas was released and none was retained within the coal seam). Following on the premise of Grocke et al. (2009), 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 be observed adjacent to the intrusions.

Using examples from coals intruded by both sills and dikes from several basins (Piceance, Illinois, and Karoo 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. Petrographically, 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 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.