The effect of Al on Si isotope fractionation investigated by silica precipitation experiments

- No Si isotope fractionation during polymerization of silicic acid
- Si isotope fractionation occurs only in the presence of Al as carrier phase.
- Formed solids re-organize as Si isotope values reaching near-initial composition.
- Si isotope fractionation is strongly dependent on net formation rate.
- Enrichment of light Si in natural solids caused by unidirectional kinetic isotope effect

Mass-dependent isotope fractionation occurring during precipitation of solids in low-temperature environments often depends on precipitation rate. Using a series of precipitation experiments in which continuous precipitation and dissolution of Si solids is forced by daily cyclic freezing (solid formation) and thawing (solid re-dissolution), we show this dependence. We conducted six Si precipitation experiments for about 120 days with initial dissolved Si concentration of 1.6 mmol/l Si, at pH values between 4.5 and 7, with additions of 0.1-1 mM of dissolved aluminum (Al), and in the absence of Al. During all experiments increasing amounts of an X-ray amorphous silica-containing solid are formed. No Si isotope fractionation occurs during formation of almost pure Si solids, interpreted as an absence of Si isotope fractionation during polymerization of silicic acid. Si isotope fractionation occurs only in the high-Al concentration experiments, characterized by an enrichment of the light Si isotopes in the solids formed early. With ongoing duration of the experiments, a re-dissolution of these solids is indicated as the Si isotope value of the complementary solution shifts to lighter values and eventually reaches near-initial compositions. Hence, our high-Al experiments are characterized by a gradual shift from a regime that is dominated by unidirectional kinetic isotope fractionation with solids formed that are up to 5‰ lighter in their 30Si/28Si ratio than the corresponding solution, to one of steady-state between dissolution and precipitation with the 30Si/28Si ratio of the solid being almost identical to the solution (Δsolid ‐ solution ≈ 0). This suggests that the enrichment of light Si isotopes found in natural environments is caused exclusively by a unidirectional kinetic isotope effect during fast precipitation of solids, aided by co-precipitation with Al phases or other carrier phases (e.g. Fe(III)). By contrast, during slow precipitation, or in the absence of a carrier phase like Al, no Si isotope fractionation is expected and solids obtain the composition of the ambient fluid.