Fractionation of silicon isotopes in liquids: The importance of configurational disorder
Résumé
Silicon isotopes are a promising tool to assess low-temperature geochemical processes such as weathering or chert precipitation. However, their use is hampered by an insufficient understanding of the fractionation associated with elementary processes such as precipitation or dissolution. In particular, the respective contributions of kinetic and equilibrium processes remain to be determined. In this work, equilibrium fractionation factors for silicon isotopes have been calculated using first-principles methods for quartz, kaolinite, and dissolved silicic acid (H4SiO4 and H3SiO4 -) at 300K.The two liquid systems are treated both as realistically as possible, and as consistently with the solids as possible. They are first simulated by ab initio molecular dynamics, then individual snapshots are extracted from the trajectories and relaxed, giving inherent structures (IS). The fractionation properties of these IS are then calculated. A significant variability of the fractionation properties (σ= 0.4‰) is observed between the independent snapshots, emphasizing the importance of configurational disorder on the fractionation properties of solutions. Furthermore, a correlation is observed between the fractionation properties of these snapshots and the mean Si-O distances, consistent with calculations on minerals. This correlation is used to identify other parameters influencing the fractionation, such as the solvation layer. It is also used to reduce the number of configurations to be computed, and therefore the computational cost.At 300K, we find a fractionation factor of +2.1±0.2‰ between quartz and H4SiO4, +0.4±0.2‰ between kaolinite and H4SiO4, and -1.6±0.3‰ between H3SiO4 - and H4SiO4. These calculated solid-solution fractionations show important disagreement with natural observations in low-temperature systems, arguing against isotopic equilibration during silicon precipitation in these environments. On the other hand, the large fractionation associated with the de-protonation of silicic acid suggests the importance of speciation, and in particular pH, for the fractionation of silicon isotopes at low temperature.
Mots clés
Stable isotope fractionations
Fractionation
isotopic fractionation
kaolinite
precipitation (chemistry)
quartz
silica
solid solution
speciation (chemistry)
stable isotope
silicic acid
silicon
Calculations
Isotopes
Molecular dynamics
Silicon oxides
Temperature
Ab initio molecular dynamics
Equilibrium fractionations
First principles method
First-principles calculation
Properties of solutions
Silica precipitation
Silicon isotopes