The impact of host-rock composition on devolatilization of sedimentary rocks during contact metamorphism around mafic sheet intrusions

Sedimentary rocks represent vast reservoirs for hydrous and carbonaceous fluids (liquid or gas) that can be generated and released during contact metamorphism following the emplacement of igneous sill intrusions. A massive release of these fluids may impose perturbations in the global climate. In this study we assess the influence of varying host-rock compositions on the magnitude and type of fluids generated from thermal devolatilization, with particular emphasis on carbon and halogens released from heated limestone, coal and rock salt, and the different timescales of metamorphism. In limestones the generated fluids are dominated by H2O with limited CH4 and CO2 production on a time-scale of 600–3000 years. Cracking of organic matter and CO2 production (8000–28,000 years) dominates the fluid products from a coal sequence. In the case of evaporites, the presence of reactive organic matter or petroleum results in the generation of CH4 and CH3Cl (260–1000 years). In order to compare the basin scale impacts of the differing host-rocks, two plausible scenarios are explored in which a 100 m thick and 50 000 km2 large sill is emplaced into 1) organic-rich shale and coal, and 2) limestones and rock salt. The results show the formation of 1) >1600 Gt CH4, and 2) >700 Gt of CH3Cl, demonstrating that the sill emplacement environment (i.e., the composition of the host rocks) is of major importance for understanding both gas generation in sedimentary basins and the environmental impact of a Large Igneous Province. By evaluation of the isotopic signature of carbonaceous fluids from shales and coals, we show that intrusions into coal-rich sediments are potentially of much less importance for perturbing the atmospheric carbon isotope values than shales.