Description: Highlights • Reaction path models quantified gas-charged waters/basalt interactions • Gas-charged freshwater and seawater compared • Geochemical reactions modelled at temperatures from 25 to 260°C • Optimal conditions for subsurface mineralization of CO2 and H2S identified Mineralization of freshwater-dissolved gases, such as CO2 and H2S, in subsurface mafic rocks is a successful permanent gas storage strategy. To apply this approach globally, the composition of locally available water must be considered. In this study, reaction path models were run to estimate the rate and extent of gas mineralization reactions during gas-charged freshwater and seawater injection into basalts at temperatures of 260, 170, 100, and 25°C. The calculations were validated by comparison to field observations of gas-charged freshwater injections at the CarbFix2 site (Iceland). The results show that more than 80% of the injected CO2 dissolved in freshwater or seawater mineralizes as Ca and Fe carbonates at temperatures ≤170°C after reaction of 0.2 mol/kgw of basalt, whereas at 260°C much lower carbon mineralization rates are observed in response to the same amount of basalt dissolution. This difference is due to the competition between carbonate versus non-carbonate secondary minerals such as epidote, prehnite, and anhydrite for Ca. In contrast, from 80 to 100% of the injected H2S is predicted to be mineralized as pyrite in all fluid systems at all considered temperatures. Further calculations with fluids having higher CO2 contents (equilibrated with 9 bar pCO2) reveal that i) the pH of gas-charged seawater at temperatures ≤170°C is buffered at ≤6 due to the precipitation of Mg-rich aluminosilicates, which delays CO2 carbonation; and ii) the most efficient carbonation in seawater systems occurs at temperatures 〈150°C as anhydrite formation is likely significant at higher temperatures.

Description: The dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and the ease in obtaining pure samples for laboratory experiments. Due to the relative simplicity of its dissolution mechanism, most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole. Olivine dissolution is controlled by the breaking of octahedral M2+-oxygen bonds at or near the surface, liberating adjoining SiO44− tetrahedra to the aqueous fluid. Aqueous species that adsorb to these bonds apparently accelerate their destruction. For example, the absorption of H+, H2O and, at some conditions, selected aqueous organic species will increase olivine dissolution rates. Nevertheless, other factors can slow olivine dissolution rates. Notably, olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid, by for example the presence and/or growth on these surfaces of either microbes or secondary phases. The degree to which secondary phases decelerate rates depends on their ability to limit access of the reactive aqueous fluids to the olivine surface. It seems likely that these surface area limiting processes are very significant in natural systems, lowering olivine surface reactivity in many environments compared to rates measured on cleaned surfaces in the laboratory. A survey of the literature suggests that the major factors influencing forsteritic olivine dissolution rates are 1) pH, 2) water activity, 3) temperature, and 4) mineral-fluid interfacial surface area. Evidence suggests that the effects of aqueous inorganic and organic species are relatively limited, and may be attributed at least in part to their influence on aqueous solution pH. Moreover, the observed decrease in rates due to the presence of secondary mineral coatings and/or the presence of microbes can be attributed to their ability to decrease olivine surface area directly exposed to the reactive aqueous fluid. As the reactivity of forsterite surfaces are spatially heterogeneous, its surface area normalized rates will tend to decrease as it dissolves even in the absence of a mineral or bacterial coating. Each of these factors limits and or influences the application of forsterite dissolution to 1) enhanced weathering efforts, 2) mineral carbonation, and 3) the low temperature generation of hydrogen or hydrocarbons via the oxidation of its divalent iron.