A method for Si isotope tracer kinetics experiments: Using Q-ICP-MS to obtain 29Si/28Si ratios in aqueous solutions

Abstract

Here we show that silicon isotope tracer kinetics experiments can be economically conducted with measurements of 29Si/28Si ratios in the experimental aqueous solutions using Quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) with sufficient precision. Although the isotope tracer method promises orders of magnitude improved detection sensitivity of reactions, its application has been hampered by the limited availability of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for Si isotope analysis. However, in Si isotope tracer kinetics experiments, dissolution rates are often derived from isotope fractional abundance increments by as much as 0.5 unit (equivalent to 50%) and the method essentially relies on the difference in isotope ratios over a period of time, which render uncertainties in < ±0.01 fraction (or ±1%) in Si isotope abundances tolerable. Laboratory-measured silicate dissolution rates typically carry uncertainties of more than ±25% and with even poorer intra- and inter-laboratory replications. These special situations of kinetics experiments circumvent the conventional concerns of using Q-ICP-MS to measure isotope ratios.To test this hypothesis, we conducted two experiments. The first used Q-ICP-MS to measure a set of mixing solutions or reference solutions with known isotope compositions. The results show that the difference is an average of ±0.009 in f29. A second test used both MC-ICP-MS and Q-ICP-MS to measure the 29Si/28Si ratios in experimental solutions of albite dissolution. Ten albite dissolution experiments were conducted at 50°C and pH 8.8. Student t-test showed that the rates calculated from the two analytical data sets have no statistically significant differences at the 95% confidence level. These results demonstrate that the Q-ICP-MS measurements of 29Si/28Si ratios produced tolerable uncertainties for determining silicate dissolution rates. However, the cost is significantly reduced and more significantly, the reduced wait time makes isotope tracer experiments more feasible. Silicate minerals make up 90% of the Earth’s crust. The kinetics of silicate-water reactions is of paramount significance to many society’s mega-environmental enterprises. This method to conduct Si isotope tracer experiments, as well as the possibility to use this approach for other non-traditional stable isotopes, will have significant impact on geochemical kinetics studies.