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

摘要

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 >5wt.%, CH_4 is the dominant volatile (85-135kg/m~3) generated through organic cracking, relative to H_2O-generation from dehydration reactions (30-110kg/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 ~2700-16200Gt CH_4 in the Karoo Basin (South Africa), and ~600-3500Gt CH_4 in the V?ring and M?re basins (offshore Norway). The generation of volatiles is occurring on a time-scale of 10-1000years 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.