Equilibrium and kinetic Si isotope fractionation factors and their implications on Si isotope distributions in the Earth’s surface environments

Abstract:

Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H₄SiO₄ solution are complemented to facilitate explanation of distributions of Si isotope in the Earth’s surface environments. The results reveal that heavy Si isotopes will be significantly enriched in the secondary silicate minerals in comparison to aqueous H₄SiO₄. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of ²⁸Si-enrichment in hyper-coordinated organosilicon complexes is found the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer is calculated and the result supports previous statement that highly ²⁸Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations.

With equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many surface systems of the Earth can be explained. For example, the change of bulk soil δ³⁰Si can be predicted as a concave pattern with respect to weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches its maximum. When well-crystallized clays start to precipitate from pore solutions under equilibrium conditions, the bulk soil δ³⁰Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain δ³⁰Si variations in the ground water profile. Equilibrium Si isotope fractionations among quadra-coordinated organosilicon complexes and the H₄SiO₄ solution may also shed the light on the Si isotope distributions in Si-accumulating plants.