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 CO 2 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, δ 13 C ratios, and δ 18 O 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 CO 2 is stored as carbonate per km 2 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.