Description: The accurate interpretation of Si isotope signatures in natural systems requires knowledge of the equilibrium isotope fractionation between Si-bearing solids and the dominant Si-bearing aqueous species. Aqueous silicon speciation is dominated by silicic acid (H 4 SiO 4o ) in most natural aqueous fluids at pH 〈 8.5, but forms H 3 SiO 4⁻ , H 2 SiO 4²⁻ , and polymeric Si species in more alkaline fluids. In this study isotope exchange experiments were performed at bulk chemical equilibrium between amorphous silica (SiO 2 ∙0.32 H 2 O) and inorganic aqueous fluids at pH ranging from 5.8 to 9.9 at 25° and 75 °C with experiments running as long as 375 days. The three-isotope method was used to quantify the equilibrium Si isotope fractionation, Δ eq³⁰ Si, between amorphous silica and aqueous Si; at pH ∼ 6 this equilibrium fractionation factor was found to be 0.45 ± 0.2‰ at 25 °C, and 0.07 ± 0.6‰ at 75 °C. At more basic pH (〉9), equilibrium Si isotope fractionation factors between solid and aqueous solution are higher, at 1.63 ± 0.23‰ at 25 °C, and 1.06 ± 0.13‰ at 75 °C. Taking account of the distribution of the aqueous Si species, equilibrium Si isotope fractionation factors between H 3 SiO 4⁻ and H 4 SiO 4o of −2.34 ± 0.13‰ and −2.21 ± 0.05‰ at 25 and 75 °C, respectively, were determined. The distinct equilibrium isotope fractionation factors of H 3 SiO 4⁻ and H 4 SiO 4o , and its variation with temperature can be used to establish paleo-pH and temperature proxies. The application of the three-isotope method also provides insight into the rates of isotopic exchange. For the solid grain size used (∼20 nm), these rates match closely the measured bulk dissolution rates for amorphous silica for most of the isotope exchange process, suggesting the dominant and rate controlling isotope exchange mechanism in the experiments is detachment and reattachment of material at the amorphous silica surface.

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.