Kinetic and equilibrium isotope effects make subtly different predictions about the mass-dependent scaling between the silicon (Si) isotope ratios ³⁰Si/²⁸Si and ²⁹Si/²⁸Si. The mass-dependencies of kinetic vs. equilibrium fractionations are typically not resolvably different during analysis, but recent work [1,2] demonstrates that with high-precision, high-resolution analyses deviations from a fractionation line are in fact quantifiable. This opens up a new analytical frontier of ‘triple silicon isotopes’ that can build on frameworks developed for e.g. the oxygen and sulfur isotope systems, to provide a new constraint on Si sources and processing. Here we present the results of preliminary experiments designed to capture a transition from kinetic to equilibrium fractionation during amorphous silica precipitation. Using educt-product (rather than standard-sample) bracketing on a Neptune HR-MC-ICP-MS, we investigate optimal measurement strategies for improved precision of triple silicon isotopes. We also present evidence for mass-independent silicon isotope fractionation during MC-ICP-MS analyses, and discuss the extent to which this might confound measurement and interpretation of triple silicon isotopes.

1. Sun et al. 2023, EPSL 607 https://doi.org/10.1016/j.epsl.2023.118069
2. Pack et al. 2023, GGG 24 https://doi. org/10.1029/2023GC011243