EFFECT OF CONTACT METAMORPHISM ON COAL GEOCHEMISTRY AND PETROGRAPHY: IMPLICATIONS FOR THE THE LARGE SCALE RELEASE OF 12C-ENRICHED METHANE

Several studies have attempted to correlate the emplacement of large igneous provinces and synchronous δ¹³C excursions within the sedimentary record with the large-scale release of ¹²C-enriched thermogenic methane produced by the intrusion of igneous rocks into organic-rich rocks. However, simple mass-balance calculations suggest that methane generated by such events would not significantly affect atmospheric carbon levels. This study asserts that if these models are to be applied to large-scale releases of ¹³C-depleted 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 (Illinois, Karoo, and Piceance), we emphasize the importance of maceral content and rank at the 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 toward the intrusion whereas 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 examined show any significant enrichment of the coal as the intrusion is approached, with shifts only on the order of 0.4‰ to slightly more 1‰, hardly what might be expected if the intrusion had resulted in a significant release of ¹³C-depleted gasses. 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 ¹³C_org-depleted volatiles leading to negative δ¹³C excursions in the sedimentary record.