The experimental determination of equilibrium Si isotope fractionation factors among H4SiO4 degrees, H3SiO4- and amorphous silica (SiO2 center dot 0.32 H2O) at 25 and 75 degrees C using the three-isotope method

Abstract

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 (H4SiO4o) in most natural aqueous fluids at 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 H4SiO4o 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 H4SiO4o, 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