NEW INSIGHTS INTO LOW-TEMPERATURE SI ISOTOPE EXCHANGE KINETICS AND FRACTIONATION USING LABORATORY EXPERIMENTS

Si isotope fractionation factors between aqueous silica and Si gel, quartz, or clay minerals predicted by theory or measured in previous experiments vary greatly, posing a significant problem for applying this isotope system to study the Si cycle in natural environments. The broad range of Si isotope fractionations measured in experiments have been interpreted to reflect kinetic isotope effects. All previous experiments have used unidirectional, synthesis approaches, based in part on the assumption that exchange experiments were not possible. Although this might be a reasonable assumption for quartz, it does not necessarily apply to all aqueous-solid systems.

We present new results from a series of experiments designed to better constrain Si isotope fractionation between aqueous silica and amorphous Si gel. Our experiments operated at aqueous Si concentration close to the solubility of Si gel to avoid precipitation or dissolution kinetics. Also, aqueous silica was spiked with ²⁹Si to quantify the rate and extent of isotopic exchange. Potential kinetic effects, which produce changes in isotope exchange trajectories with time, were evaluated using different initial δ³⁰Si values, ranging from +11.5‰ to -7.4‰, at low and high ionic strengths. Depending on the contrast in initial δ³⁰Si values of the two phases, Si gels became enriched or depleted in light Si isotopes, showing equilibrium Si isotope fractionation in our experiments. Si isotope exchange reached ~50% to ~80% relative to equilibrium within 2 months, and almost ceased thereafter. The extent of Si isotope exchange was primarily controlled by the surface area of Si gels and ionic strengths, with higher surface area and ionic strengths corresponding to higher exchange extent. The equilibrium fractionation factor between Si gel and aqueous Si is estimated to be ~0‰ at room temperature, which is significantly different from the large ~-2 ~ -3‰ fractionations between Fe-Si gel and aqueous silica estimated in our previous study [1]. Our results show that amorphous Si and Fe-Si gels exchange isotopes readily and are therefore likely to attain isotopic equilibrium in natural systems, and we further show that Fe-Si bonding has a profound effect on stable Si isotope fractionations.

[1] Zheng et al., GCA, 2016