Concurrent volcanic and thermogenic emissions from a large mafic complex intersecting a petroleum system in the Oslo Rift, Norway

The interaction between magmas and organic matter from host sedimentary rocks may lead to ore mineralization, hydrocarbon cracking and thermogenic gas emissions, with carbon cycle perturbations. Here, we explored the nature of interaction processes involving basaltic melts, bitumen- and oil-bearing sandstones in a continental rift setting, combining field observations and geochemical analyses. Our case study is an island in the Oslo Fjord (Norway), where numerous dykes and a sill likely compose the magma plumbing system of an adjacent basaltic volcano, as part of the 300 Myr-old Skagerrak Large Igneous Province, and pyrobitumen occurs in sandstone xenoliths and breccias within the sill. Our multi-technique approach characterized the transformation of bitumen and oil into pyrobitumen along with abundant methane (CH4) and ethane (C2H6) emissions. Our geochemical dataset described element exchange and mass transfer between melts, fluids and host rocks, from magmatic to hydrothermal stages, depicting a scenario with magmatic intrusions of a large mafic complex intersecting a pre-existing petroleum system. Bile Island yields an extraordinary record, where magmatic and sedimentary carbon is synchronously released via the same volcanic vent(s), offering a viable explanation for methane emissions in volcanic areas and a new paradigm for degassing in the context of Large Igneous Provinces.