Pore-fluid pressure assisted columnar joint initiation: a natural carbon trapping process revealed from IODP Expedition 396 basalt samples

Extensive flood basalt sequences of the North Atlantic Igneous Province (NAIP) offshore Norway comprise promising sequestration reservoirs for permanent CO₂ on the doorstep of Europe. However, a key question remains open: How much of the pore space is available at present for Carbon Capture and Sequestration (CCS) endeavours?

We investigated carbonate vein and void mineral fills in drill core samples from IODP Expedition 396 which recovered over 350 m of basaltic stratigraphy, to untangle the formation conditions of the carbonates and the origin of the carbon. We examined two main scenarios: carbonates originating from 1) dissolved carbon in seawater, or from 2) thermogenic gas ascending from greater depths within the basin. Several driving mechanisms for this second scenario are explored: natural migration of light petroleum derived from burial and/or contact metamorphism associated with later sill emplacement at depth.

We present results of carbon and oxygen isotope analyses of carbonate found distributed down section of the landward-flow volcanic sequence (Site U1566) and the outer seaward-dipping reflector (SDR) sequence of flows (Site U1573). Oxygen isotope thermometry suggests a range of formation temperatures between 50 and 160°C. However, results from micro-CT textural analyses and petro-chemical analyses suggests that some carbonate originated from thermogenic gas at much higher temperature. We interpret these signatures as relating to the early nucleation of contractional cooling joints (columnar joint fractures) in the basaltic lava at the time of emplacement. Newly obtained carbon and oxygen isotopes from this type of carbonate will be presented and discussed.

Our result suggests an alternative third scenario in which carbonate originating from below the flood basalts seals the porosity and reduces permeability in the lower section of the SDR sequence. Textural analysis indicates that thermogenic gases mobilized during lava flow emplacement assist in initiating and sealing columnar joint fractures. If confirmed, this process implies that the SDR lava flow sequence above the first lava flow may be relatively carbonate-free, as it is sealed off from post-emplacement thermogenic gas fluxes ascending from depth.