How contact metamorphism can trigger global climate changes: Modeling gas generation around igneous sills in sedimentary basins

• Published in: Geochimica et cosmochimica acta 2010-12, Vol.74 (24), p.7179-7195
• Main Authors: Aarnes, Ingrid, Svensen, Henrik, Connolly, James A.D., Podladchikov, Yuri Y.
• Format: Article
• Language: English
• Subjects: Air pollution; Basins; Contact; Dehydration; Devolatilization; Greenhouse effect; Intrusion; Mathematical models
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.09.011

Large volumes of greenhouse gases such as CH 4 and CO 2 form by contact metamorphism of organic-rich sediments in aureoles around sill intrusions in sedimentary basins. Thermogenic gas generation and dehydration reactions in shale are treated numerically in order to quantify basin-scale devolatilization. We show that aureole thicknesses, defined as the zone of elevated metamorphism relative to the background level, vary within 30–250% of the sill thickness, depending on the temperature of the host-rock and intrusion, besides the sill thickness. In shales with total organic carbon content of >5 wt.%, CH 4 is the dominant volatile (85–135 kg/m 3) generated through organic cracking, relative to H 2O-generation from dehydration reactions (30–110 kg/m 3). Even using conservative estimates of melt volumes, extrapolation of our results to the scale of sill complexes in a sedimentary basin indicates that devolatilization can have generated ∼2700–16200 Gt CH 4 in the Karoo Basin (South Africa), and ∼600–3500 Gt CH 4 in the Vøring and Møre basins (offshore Norway). The generation of volatiles is occurring on a time-scale of 10–1000 years within an aureole of a single sill, which makes the rate of sill emplacement the time-constraining factor on a basin-scale. This study demonstrates that thousands of gigatons of potent greenhouse gases like methane can be generated during emplacement of Large Igneous Provinces in sedimentary basins.