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

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 (Δ r G) 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 Δ r G 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 Δ r G range, signaling no major Δ r G -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 H 2 O-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 H 2 O-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. [ABSTRACT FROM AUTHOR]