GEOCHEMICAL, ISOTOPIC, AND PETROGRAPHIC ANALYSIS OF PERMIAN COALS AND CARBONACEOUS SHALES FROM ANTARCTICA: IMPORTANCE OF RANK AND MACERAL COMPOSITION

Large-scale methane release resulting in global atmospheric change has been proposed as one possible cause for the end-Permian mass extinction. It has been suggested that an expansive methane release resulted from widespread igneous intrusion of coals and carbonaceous rocks. Possible evidence for this release includes d13C excursions in Permian-age sediments including those in the organic-rich rocks in Antarctica. Isotopic composition of organic matter (both coals and organic-rich shales) varies based on a number of factors; however, maceral composition (i.e., type of organic components) and rank (level of organic maturation) are often overlooked and yet these can be important factors. In order to adequately assess isotopic trends across the Permo-Triassic boundary, it is important to understand how these particular factors may influence organic matter isotopic composition in Antarctic sedimentary rocks. Petrographic analysis of samples collected from the Allan Hills, Graphite Peak, and Coalsack Bluff sites in Antarctica show that the coals/carbonaceous shales are high in rank, with vitrinite reflectances ranging from 2% to over 8%. Even though care was taken in the field to collect samples that would be least influenced by intrusion, some show evidence of natural coking whereby vitrinite (derived from woody tissues) has gone through a plastic phase and has developed optical anisotropy. Liptinites are lost (volatilized in the process) and inertinites remain to a greater or lesser degree unaltered (hence the name inertinite) during contact metamorphism. Pyrolytic carbon (material deposited from a vapor phase) may also be present in some samples. Proximate and ultimate analyses of these Antarctic samples show anomalous results. Volatile matter contents can be influenced by carbonate mineralization, and preliminary results from a subset of HCl-treated samples suggest a contact metamorphic imprint. Reflectance-VM relationships of organic matter altered by intrusions are different from those of organic matter that undergoes normal burial coalification; data from these rocks indicate contact metamorphism.