Measuring silicate mineral dissolution rates using Si isotope doping

Abstract

New experimental data and quantitative models show that the 29Si doping experimental technique (Gruber, Zhu, and others, 2013, GCA) is robust for measuring silicate mineral dissolution rates even while a Si-containing secondary phase is precipitating. In this study, batch experiments of albite dissolution were conducted under ambient temperature and pH3–7.5, some seeded with kaolinite. Initial solutions of various Si concentrations were doped with 29Si, resulting in a Si isotopic composition highly anomalous to natural Si isotope compositions. The isotopic contrast and precision of isotope fraction analysis to ±0.0005 to ±0.001 allow detection of the dissolution of a minuscule amount of albite in aqueous solutions. Experimental data and quantitative modeling show Si isotope fractionation during albite dissolution ranged from 30εsol-ab −2.870 to 0.804‰, significant for Si biogeochemical cycling, but resulting in only <±0.04% errors in rate determination. The simultaneous precipitation of secondary phases consumed silica, causing slight changes of Si isotope ratios, but the isotopic fractionation due to secondary phase precipitation is negligible for determining albite dissolution rates. Combination of Si isotopes and Si concentrations, precisely measured with the Si isotope dilution method, allowed determination of secondary phase precipitation rates simultaneously. This means that we can now measure rates at circumneutral pH and near equilibrium conditions, even when secondary precipitates are forming. However, while the isotope doping method has greatly improved the precision and sensitivity of rate measurements, the accuracy of rate measurements is still subject to the vagaries of sample preparation and other unknown effects as shown our data near pH5.5. When the solution is very close to equilibrium, the backward reaction becomes important and interpretation of the isotope data would be complicated or impossible.