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

Large volumes of greenhouse gases such as CH4 and CO2 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.%, CH4 is the dominant volatile (85–135 kg/m3) generated through organic cracking, relative to H2O-generation from dehydration reactions (30–110 kg/m3). 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 CH4 in the Karoo Basin (South Africa), and ∼600–3500 Gt CH4 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.