Anatomy of an intruded coal, II: effect of contact metamorphism on organic δ13C and implications for the release of thermogenic methane, Springfield (No. 5) Coal, Illinois Basin

Abstract

Concerns about the present-day accelerated rate of carbon input to the atmosphere have prompted the search for comparable events throughout Earth's history. Recent research has attempted to determine the cause of such events, represented as negative δ13C excursions within the sedimentary record, and their subsequent effect on the Earth. It has been suggested 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 (LIPs) into organic-rich rocks and/or coal (e.g., McElwain et al., 2005; Svensen et al., 2004, 2007). Our study asserts that if intrusions result in significant release of 13C-depleted thermogenic methane from intruded coals, 13C-enriched coal should be observed adjacent to the intrusions.In this study, values of bulk organic δ13C (δ13Corg) are reported for fifteen samples along a coal transect in the Illinois Basin where a Pennsylvanian-age coal (the Springfield or No. 5 Coal) is intruded by a Permian ultramafic dike. The samples reveal significant bulk geochemical and petrographic changes within the alteration halo; for example, volatile matter (VM) decreases towards the intrusion, whereas fixed carbon (FC) and vitrinite reflectance (Rr) both increase, accompanied by the development of natural coke (both isotropic and anisotropic), mosaic structure, devolatilization vacuoles, and carbonate mineralization. Reflectance levels and coke development are consistent with exposure to temperatures >450–500°C. Despite the high level of alteration of the organic matter, no significant change is observed in δ13Corg; values across the transect vary by less than 0.5‰. Along this transect, δ13Corg does not correlate with Rr and does not appear to be influenced by maceral composition. The intruded coals show no evidence of pyrolytic carbon that could influence δ13Corg.The relatively high VM contents of the intruded coals (compared to coals that have undergone normal burial maturation) and the lack of any trend in δ13Corg approaching the intrusion raises questions about previously published models of the effects of large-scale igneous intrusions on atmospheric composition and δ13Corg excursions associated with oceanic anoxic events. This study underscores the importance of the use of organic petrography in combination with isotope geochemistry to determine if contact metamorphosed coals and organic-rich shales could have released a significant amount of 13Corg-depleted volatiles leading to negative δ13Corg excursions in the sedimentary record.