Mineralogical aspects of CO ₂ sequestration during hydrothermal basalt alteration - An experimental study at 75 to 250°C and elevated pCO ₂

The lteration of basaltic glass was studied experimentally at elevated CO ₂ pressures (~10-25bar) and hydrothermal conditions (75-250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO ₂-water-basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite-ankerite solid solutions (Ca-Mg-Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca-Mg-Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca-Mg-Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO ₂ conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al-Si-mineral formation. At <100°C, the divalent cations are available for Ca-Mg-Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO ₂ at <100°C associated with the formation of Ca-Mg-Fe carbonates.