No evidence for thermogenic methane release in coal from the Karoo-Ferrar large igneous province |
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| Authors: | Darren R Gröcke Susan M Rimmer Lois E Yoksoulian Bruce Cairncross Harilaos Tsikos Jeroen van Hunen |
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| Institution: | 1. Environmental Resources and Policy Program, Southern Illinois University Carbondale, Carbondale, IL 62901, United States;2. Department of Geology, Southern Illinois University Carbondale, Carbondale, IL 62901, United States;1. Sorbonne Université, CNRS, Institut des Sciences de la Terre Paris, ISTeP, F-75005 Paris, France;2. ASD/IMCCE, CNRS-UMR8028, Observatoire de Paris, PSL University, Sorbonne Université, 77 Avenue Denfert-Rochereau, 75014 Paris, France;3. Moulay Ismaïl University, Faculty of Science, Department of Geology, BP. 11201 Zitoune, 50070 Meknes, Morocco;4. Sorbonne Université, MNHN, CNRS, Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements, CR2P, F-75005 Paris, France;1. UMR CNRS 5276 LGLTPE, Université Lyon 1, Campus de la Doua, Bâtiment Géode, F-69622 Villeurbanne Cedex, France;2. Departamento de Ciências da Terra and IMAR-CMA, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Largo Marquês de Pombal, 3000-272 Coimbra, Portugal |
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| Abstract: | The Early Jurassic Toarcian oceanic anoxic event (T-OAE) and concurrent negative carbon-isotope (δ13C) excursion have recently been attributed to either the release of methane (CH4) clathrates or thermogenic CH4 gas associated with the Karoo-Ferrar large igneous province (LIP) into coals and organic-rich shales. 12C-enriched thermogenic CH4 production associated with the Karoo-Ferrar would result in residual material being 12C-depleted nearer the intrusions. In this study, geochemical analyses (carbon isotopes, volatile matter (VM), vitrinite reflectance (Ro)) are reported for two coal transects associated with dykes intruding the No. 4L coal in the Highveld Coalfield, Karoo Basin, South Africa. VM decreases from over 35% to around 15% in one transect, and the second transect shows a less pronounced decrease (from > 25% to ~ 16%). Accompanying the decrease in VM content is an increase in Ro from background levels of around 0.7% to over 4% adjacent to the dyke; used as a palaeo-geothermometer, Ro values indicate background temperatures of ~ 100 °C increasing to > 300 °C close to the contact. Despite changes in VM and Ro, there are no significant changes in δ13C, certainly not of the magnitude that would be expected associated with large-scale thermogenic CH4 generation. These and other Gondwanan coals have low vitrinite and liptinite contents (components more prone to CH4 generation), in part explaining the modest decreases in VM adjacent to the dykes. This, combined with the relatively narrow metamorphic aureole surrounding the intrusions and the likelihood that at least some of the volatiles generated by the intrusion were trapped as coalbed CH4 or condensed as pyrolytic carbon, suggests only limited CH4 release. In addition, based on original estimates of moisture contents in these coals and the depth at time of intrusion (1,000–2,000 m) the dykes would have lost most of their energy heating and evaporating water, thus having very little remaining energy to generate thermogenic CH4. |
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