How contact metamorphism can trigger global climate changes: Modeling gas generation around igneous sills in sedimentary basins |
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| Authors: | Ingrid Aarnes Henrik Svensen Yuri Y Podladchikov |
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| Institution: | a Physics of Geological Processes (PGP), Department of Physics, University of Oslo, P.O. Box 1048 Blindern, Norway
b Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland |
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| Abstract: | 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. |
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