A New Method for Silicon Triple Isotope Analysis With Application to Siliceous Sponge Spicules

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2023-12, Vol.24 (12), p.n/a
• Main Authors: Pack, Andreas, Bultmann, Eike‐Matthias, Tatzel, Michael, Reitner, Joachim
• Format: Article
• Language: English
• Subjects: Chemical analysis; Diatomites; Fossils; Fractionation; Isotopes; Lasers; Mass spectrometry; Mass spectroscopy; Ratios; Scientific imaging; Seawater; Silica; Silicic acid; Silicon; Silicon isotopes; Sponges; triple isotopes; Water analysis
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011243

Multiple isotope ratios of elements disclose information on the fractionation mechanism that cannot be obtained from a single isotope ratio alone. We describe a laser fluorination technique in combination with gas source mass spectrometry for the measurement of triple Si isotope ratios with high precision that allows resolving ppm‐level variability in triple silicon isotope ratios (29Si/28Si, 30Si/28Si) Δ‘29Si due to purely mass‐dependent fractionation in Si isotopes. We demonstrate how the triple silicon isotope ratios can be used to characterize different mass‐dependent processes that fractionate silicon isotopes. We report new data on reference materials (NBS‐28 [RM8546], Diatomite, BigBatch) and on siliceous sponges. Our triple silicon isotope data resolve that kinetic fractionation, possibly related to breaking Si(OH)4 molecules, causes the low δ30Si of sponges. The data further suggest that the cause for fractionation may be the same for the groups of Demospongiae, Homoscleromorpha, and Hexactinellida and that triple silicon isotope ratios of sponges have potential as seawater silicon isotope proxy. Plain Language Summary In this study, we present a technique for high‐precision measurements of the triple silicon isotope ratios (29Si/28Si, 30Si/28Si, expressed as δ29Si and δ30Si) of silica. Silica (∼2–3 mg) samples were heated with an infrared laser in an atmosphere of BrF5 gas. The released SiF4 gas was analyzed by gas source mass spectrometry. We present data on standards and various sponge silica spicules. Small variations in the δ29Si and δ30Si can be resolved in the standards and the sponge spicules. Such variations provide information on the fractionation mechanism of silicon isotopes. In our study, we suggest that the sponges kinetically fractionation silicon during the incorporation of dissolved silicic acid into the silica spicules. We suggest that, with improved precision, seawater δ30Si values can be deduced from the analysis of fossil sponge spicules. Key Points We present a new method of triple silicon isotope analysis We present new data on standards and sponge spicules We suggest that the three silicon isotopes allow reconstruction of seawater silicon isotopes from sponge spicule data