Natural CO2 sequestration: mechanisms and conditions of carbonate precipitation in basalt

Carbon sequestration in mafic and ultramafic rocks has been demonstrated as a safe and efficient storage method; however, the specifics of the carbonation reactions remain uncertain, especially in volcanic reservoirs. In this study, we contribute to a deeper understanding of the mechanisms and conditions of natural CO2 sequestration in basalt by analysing 43 early Eocene basalt samples from IODP boreholes U1571A and U1572A on the Skoll High on the mid-Norwegian continental margin. Using high-resolution 2D and 3D imaging and microanalytical techniques together with X-ray powder diffraction, U-Pb dating, as well as carbon and oxygen isotope analysis, we identified two main types of carbonate precipitation: 1) direct precipitation in vesicles, and 2) replacement of olivine and clinopyroxene by coupled dissolution-precipitation reactions. While most vesicles are coated with smectite, this did not inhibit fluid flow and carbonate precipitation. Mineral parageneses, δ13C ratios, and δ18O ratios record carbonate precipitation in the vesicles at < 100˚C and suggest a variation of carbon sources, from early intruded meteoric water mixed with volcanic and thermogenic carbon to marine sourced carbon. Vesicle carbonate comprises up to 8.6 vol% of its host basalt (as determined by μ-CT image analysis) and replacement carbonate 0.6 – 0.94 wt% (as determined by powder diffraction). We estimate that within the upper 100 m of the subaerially emplaced basalt sequence on Skoll High, approximately 1 Mt of CO2 is stored as carbonate per km2 area, at least 0.3 Mt of which has precipitated in the microporous basalt through coupled dissolution-precipitation reactions. This illustrates that the storage potential of basaltic rocks is not limited to the macro-porosity (vesicles) but may be significantly enhanced by replacement reactions.