A first look at Ge/Si partitioning during amorphous silica precipitation: Implications for Ge/Si as a tracer of fluid-silicate interactions

We measured germanium-silicon (Ge/Si) ratios in both fluid and solid phases using a series of highly constrained amorphous silica precipitation experiments at 20 °C and neutral pH for a wide range of seed crystal surface areas. Silicon isotope data (δ^30/28Si) for these experiments were previously reported by Fernandez et al. (2019). A distinct lag in the onset of Ge/Si partitioning relative to δ³⁰Si fractionation during active amorphous silica growth indicated that Ge incorporation rates were orders of magnitude slower than silicon precipitation rates. Slow Ge kinetics give the appearance of conservative behavior over short experimental timescales (∼30 days). A major outcome from these observations was the existence of distinct equilibration timescales between Ge/Si and δ³⁰Si. Further, these experimental results provide the first documented evidence of rate dependent behavior in Ge partition coefficients. Successful application of a modified solid solution model, initially developed for characterizing stable isotope fractionation, indicates that a common fractionation model theory is able to describe both Ge/Si and δ³⁰Si partitioning. Numerical simulations conducted at longer timescales (1–1000 years) predict eventual Ge incorporation into the mineral surface, but this occurs when the system is in close proximity to equilibrium conditions. These long-term predictions underscore the potential of Ge as a (near)equilibrium tracer in complement to the mixed kinetic and equilibrium signatures recorded by δ³⁰Si. Our findings illustrate the viability of a combined Ge/Si–δ³⁰Si multi-tracer approach for constraining silicate mineral formation across a variety of terrestrial and marine systems.