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/m(3)) generated through organic cracking, relative to H2O-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 similar to 2700-16200 Gt CH4 in the Karoo Basin (South Africa), and similar to 600-3500 Gt CH4 in the Wring and More 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. (C) 2010 Elsevier Ltd. All rights reserved.