Low Temperature Reaction Experiments Between Basalt, Seawater and CO2, and Implications for Carbon Dioxide Sequestration in Deep-Sea Basalts

Reactions between divalent cation-rich silicate minerals and CO2-bearing fluids to form (Ca, Mg, Fe) carbonate minerals could facilitate the safe and permanent storage of anthropogenic carbon dioxide. Deep-sea basalt formations provide large storage reservoir capacities and huge potential sources of Ca2+, Mg2+ and Fe2+. However, better knowledge of silicate mineral reaction rates with carbonate-bearing fluids is required to understand the overall carbon storage potential of these reservoirs. This study investigates key reactions associated with progressive seawater-rock interaction using far-from equilibrium dissolution experiments. The experiments were carried out at 40 ˚C and at constant CO2 partial pressure of 1 atm. Mid-ocean ridge basalts from the Juan de Fuca and Mid-Atlantic Ridges and a gabbro from the Troodos ophiolite were reacted with 500 mL of CO2-charged seawater using thick-walled fluorinated polypropylene bottles combined with rubber stoppers. The starting material was crushed, sieved and thoroughly cleaned to remove fine particles (< 63 μm) to ensure a particle grain size between 63 and 125 μm for all the samples. The seawater chemistry and the pH were monitored throughout the experiments by daily analysis of 1 mL of fluid. The pH increased rapidly from 4.8 to 5.0 before stabilizing at 5.1 after 10 days of reaction time. The analysis of anions (S, Cl) highlighted a substantial evaporation (up to 15 %) during the experiments, requiring a correction factor for the measured dissolved ion concentrations. Evaporation corrected silicon (Si) and calcium (Ca) concentrations in the seawater increased by 5900 % and 14 %, resulting in total dissolved Si and Ca from basalt of 0.3 % and 2.4 %, respectively. The results are comparable with literature data for fresh water experiments conducted on basaltic glass at higher temperature or pressure, illustrating the considerable potential of the mineral sequestration of CO2 in submarine basalts.