Testing hypotheses of albite dissolution mechanisms at near-equilibrium using Si isotope tracers

Abstract

Here, we demonstrate the potential advantages of using isotope tracers to test hypotheses of reaction mechanisms near-equilibrium. Using non-traditional stable Si isotopes as tracers, we measured albite unidirectional dissolution rates (r+) across a range of Gibbs free energy of reaction (ΔrG) close to equilibrium (−26 to −2 kJ/mol). Thirteen batch experiment series were conducted at 50 °C and pH ∼ 8 ± 0.25. Different distances from equilibrium were achieved by a stepwise increase of concentrations of Si (0–600 μM), Al (0–10 μM), and Na (0–1000 μM). The temperature, pH, sample preparation, and reaction duration were kept identical to isolate the ΔrG effect. Secondary phase precipitation, which is difficult to avoid in near-equilibrium, near-neutral pH experiments renders the rate measurement method based on changes in Si and Al concentration unworkable, but it should not impact the Si isotope ratios-based rates.The resulting r+ values were nearly constant in the experimental ΔrG range, signaling no major ΔrG-related switch of reaction mechanisms. Our results suggest that the switch from etch pit opening at far-from-equilibrium to step retreat at near-equilibrium does not operate under circum-neutral pH in low-temperature systems; this mechanism switch was proposed based on experimental data in alkaline solutions at hydrothermal temperatures. The nearly constant r+ values at pH 5–8 also suggest that an H2O-catalyzed reaction mechanism dominant at circumneutral pH, in addition to the H+- and OH–-catalyzed reaction mechanisms dominant at acidic and alkaline pH, respectively.The experimental results have implications for geochemical modeling of low-temperature geological and environmental processes. The results suggest that a term of H2O-catalyzed reaction mechanism should be included in rate laws and that the parallel rate law with a mechanism-switch is not applicable in the pH range of 5–8.