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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 >5 wt.%, CH 4 is the dominant volatile (85-135 kg/m 3 ) generated through organic cracking, relative to H 2 O-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 $2700-16200 Gt CH 4 in the Karoo Basin (South Africa), and $600-3500 Gt CH 4 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.

Subvolcanic intrusions in sedimentary basins cause strong thermal perturbations and frequently cause extensive hydrothermal activity. Hydrothermal vent complexes emanating from the tips of transgressive sills are observed in seismic profiles from the Northeast Atlantic margin, and geometrically similar complexes occur in the Stormberg Group within the Late Carboniferous-Middle Jurassic Karoo Basin in South Africa. Distinct features include inward-dipping sedimentary strata surrounding a central vent complex, comprising multiple sandstone dykes, pipes, and hydrothermal breccias. Theoretical arguments reveal that the extent of fluid-pressure build-up depends largely on a single dimensionless number (Ve) that reflects the relative rates of heat and fluid transport. For Ve >> 1, ‘explosive’ release of fluids from the area near the upper sill surface triggers hydrothermal venting shortly after sill emplacement. In the Karoo Basin, the formation of shallow (< 1 km) sandstone-hosted vents was initially associated with extensive brecciation, followed by emplacement of sandstone dykes and pipes in the central parts of the vent complexes. High fluid fluxes towards the surface were sustained by boiling of aqueous fluids near the sill. Both the sill bodies and the hydrothermal vent complexes represent major perturbations of the permeability structure of the sedimentary basin, and are likely to have long time-scale effects on its hydrogeological evolution.

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Earth would be uninhabitable if water was not returned to exogenous reservoirsat subduction zones, preventing global ocean drainage. Yet the bottleneck mechanism that couples initial fluid release from subducting, zero-porosity rocks with chemically bound water to rocks with high-permeability fluid escape channels is unknown. Using multiscale rock analysis combined with thermodynamic modelling we show that fluid flow initiation in dehydrating serpentinites is controlled by intrinsic chemical heterogeneities, localizing dehydration reactions at specific microsites. Porosity generation is directly linked to the dehydration reactions and resultant fluid-pressure variations force the reactive fluid release to organize into vein networks across a wide range of spatial scales (µm to m). This fluid channelization results in large-scale fluid escape with sufficient fluxes to drain subducting plates. Moreover, our findings suggest that antigorite dehydration reactions do not cause instantaneous rock embrittlement, often presumed as the trigger of intermediatedepth subduction zone seismicity.