Achieving ppm-Level Precision and Accuracy in Triple Silicon Isotope Measurements by Gas Source Isotope Ratio Mass Spectrometry

Abstract

Triple silicon isotopes, expressed as Δ’29Si, the deviation of δ’29Si from the high-temperature equilibrium mass-dependent fractionation reference, provide an additional dimension for distinguishing fractionation processes that can produce overlapping single-isotope (δ’30Si or δ’29Si) signatures, making them useful for investigating terrestrial planet accretion and silicon cycling on Earth. However, their broader application has been limited by the analytical precision and accuracy required to resolve the subtle Δ’29Si variations observed in natural samples. Here, we present an analytical protocol that achieves an external reproducibility of ∼1 ppm (1 SD) for Δ’29Si using gas source isotope ratio mass spectrometry (Thermo Fisher Scientific Ultra HR-IRMS). Silicates are quantitatively converted to BaSiF6 and subsequently thermally decomposed to SiF4 for isotopic analysis. This approach avoids strong oxidizing fluorination reagents and minimizes potential sources of sulfur-bearing fluorination by-products. Systematic SO2F2 mixing experiments demonstrate that the diagnostic m/z 88 ion can be used as an empirical quality-control parameter for identifying measurements affected by sulfur-bearing isobaric interference at m/z 86. Long-term zero-enrichment tests and full-procedure analyses of reference materials demonstrate reproducibility of approximately 1 ppm for Δ’29Si. Application to three Mesoproterozoic chert samples from the Jixian Group yields Δ’29Si values of −4.1 ± 0.3 to −16.9 ± 1.8 ppm and a δ’30Si−δ’29Si regression slope of 0.5104 ± 0.0008 (1 SD), broadly consistent with previously reported Jixian chert compositions. The analytical protocol provides a practical approach for resolving subtle triple silicon isotope variations in natural geological materials, supporting future geochemical and planetary studies.