Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies

Water-rock interactions influence the isotopic signatures archived during secondary mineral formation and thus the fidelity with which these geochemical proxies retain pristine geochemical records over geologic timescales. The extent to which these isotopic signatures are “re-equilibrated” or “reset” has been suggested to be dependent on surface area and the depth into mineral surface that the surrounding fluid can access, but this relationship has yet to be incorporated in a generalized modeling framework. In this study, we evaluate the timing and extent of silicon isotopic re-equilibration during amorphous silica formation, a common secondary mineral in weathering environments, through a combination of both laboratory experiments and multi-component numerical modeling. A series of amorphous silica precipitation experiments were conducted over a range of surface areas and solid to fluid ratios at 20 °C and near neutral pH for a period of 30 days. These experiments show evidence that surface area exerts a first order control on the extent of isotopic re-equilibration following secondary mineral precipitation. New mineral growth onto higher surface areas seeds resulted in rapid isotopic re-equilibration (over a period of ∼2 weeks) whereas precipitation onto lower surface area grains retained a kinetic signature due to overall slower kinetics. Experimental results were further supported by a modified numerical reaction network model that has the novel capacity to explicitly treat the depth into the mineral that the fluid can chemically and isotopically exchange with through time. These numerical simulations demonstrate that the fluid-mineral surface interaction depth is larger for low surface area seed crystals where both a larger mass of amorphous silica was formed, and slower precipitations rates kept the reaction far from equilibrium, thereby impeding the influence of the back reaction and corresponding isotopic re-equilibration. These results highlight the significant role that water-rock interaction and, specifically, the mineral surface plays in the preservation of isotopic signatures within secondary silicate phases. Further, we suggest that a transient model approach is necessary to improve quantitative interpretations of observed isotope behavior in natural systems.