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

Abstract:

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.