ISOTOPE TRACER EXPERIMENTAL METHOD FOR MEASURING SILICATE REACTION RATES AND NEW INSIGHTS FOR GEOCHEMICAL MODELING

We used the Si isotope and measured silicate reaction rates at ambient temperature, near neutral pH, and close to equilibrium conditions, which are unattainable with the conventional Si concentration-based method. The isotope tracer technique introduces a rare isotope of an element to an experimental solution that is interacting with a mineral or a suite of minerals that have normal or natural isotopic compositions (Gaillardet, 2008; Gruber et al., 2013). Although the method itself is not new, MC-ICP-MS technology now makes the measurement of a large number of non-traditional isotopes accessible, presenting a new opportunity to utilize the isotope doping method to advance the field of geochemical kinetics.

Results from experiments for albite (Zhu et al., 2016), quartz (Liu et al., 2016), K-feldspar, and kaolinite demonstrated that the isotopic contrast and analytical precision allow detection of the dissolution of a minute amount of silicate. Because the precipitation of Si-containing secondary phases or the reverse reaction consume silica but leaves the ²⁹Si /²⁸Si ratios essentially unchanged in experimental solutions, dissolution rates were still measurable when secondary phase precipitation took place in experiments. The availability of unidirectional reaction rates permitted testing of the applicability of the Principle of Detailed Balance (PDB) for quartz and multi-oxide silicates feldspars and kaolinite.

Liu et al. (2016). Geochemical Perspectives Letters 2, 78-86.

Zhu et al. (2016). Chemical Geology 445, 146-163.