The experimental determination of equilibrium Si isotope fractionation factors among H₄SiO₄^o, H₃SiO₄⁻ and amorphous silica (SiO₂·0.32 H₂O) at 25 and 75 °C using the three-isotope method

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 (H4SiO4 o) in most natural aqueous fluids at pH < 8.5, but forms H3SiO4 −, H2SiO4 2−, and polymeric Si species in more alkaline fluids. In this study isotope exchange experiments were performed at bulk chemical equilibrium between amorphous silica (SiO2∙0.32 H2O) 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, Δeq 30Si, 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 H3SiO4 − and H4SiO4 o 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 H3SiO4 − and H4SiO4 o, 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.