Measuring silicate mineral dissolution rates using Si isotope doping

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• 作者：Chen Zhu^a ; ^{chenzhu@indiana.edu" class="auth_mail" title="E-mail the corresponding author} ; Zhaoyun Liu^a ; ^b ; Yilun Zhang^c ; Chao Wang^a ; Augustus Scheafer^a ; Peng Lu^a ; ¹ ; Guanru Zhang^a ; R. Bastian Georg^d ; Hong-lin Yuan^e ; J. Donald Rimstidt^f
• 关键词：Kinetics ; Feldspar ; Si isotope ; Reaction rates
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：16 December 2016
• 年：2016
• 卷：445
• 期：Complete
• 页码：146-163
• 全文大小：2300 K

文摘

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 pH 3–7.5, 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 ³⁰ε_sol-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 pH 5.5. When the solution is very close to equilibrium, the backward reaction becomes important and interpretation of the isotope data would be complicated or impossible.