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Stephane Polteau, University of Oslo (Norway)
Henrik Svensen, University of Oslo (Norway)
Sverre Planke, Volcanic Basin Petroleum Research (Norway)
Ingrid Aarnes, University of Oslo (Norway)
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Magmatic sill intrusions emplaced in carbonaceous sediments involve the sudden exposure of organic matter to temperatures as high as 1200°C in contact aureoles. Venting of metamorphic gases to the atmosphere represents one of few models based on geological evidence for rapid climate change and global carbon cycle perturbations.
The aims of this contribution are (1) to evaluate the thermal alteration effects on the carbon isotopic composition of organic-rich sediments by comparing thermally affected and unaffected borehole sections, (2) to calculate the carbon isotopic composition based on actual isotope data and (3) to calculate the mass of methane generated by contact metamorphism in a contact aureole and on basin scale. Large amounts of magma were injected into the Karoo Basin in South Africa 183 Ma ago. The organic-rich Ecca Group forms the base of the Karoo sedimentary succession and contains hundreds of degassing pipes structures rooted in the sill intrusions. Previous numerical and analogue modelling shows that these piercement structures form during violent explosions releasing the overpressure resulting from metamorphic devolatilization in contact aureoles surrounding the sill intrusions.
A series of bulk-geochemical analyses (total organic carbon, organic maturity and stable carbon isotopes) were carried out both borehole sections. The results show that the aureole thickness is best defined by organic matter maturity. With decreasing distance from the sill intrusion contacts, the thermal maturity increases, the total organic carbon decreases and the δ13C measured on bitumen is enriched in 13C. These results demonstrate that the organic carbon escapes the contact aureole during devolatilization processes involving the generation of light carbon gases.
The calculated isotopic composition of the carbon released is similar whether using the batch devolatilization or the Rayleigh distillation model, and ranges from the background values to 1-2‰ lighter values with decreasing distance from the contact. The extrapolation of our results to the entire sedimentary basin suggests that contact metamorphism of organic-rich sediments, synchronous with the formation of the Karoo continental flood basalts, triggered massive release of light carbon to the atmosphere, thus perturbing the global carbon cycle at the beginning of the Toarcian.
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