22-EPN3-063: The Origin of Early Archean Barite – Insights from the Geochemical and Isotopic Composition of Associated Chert Deposits
Visit by Desiree Roerdink (University of Bergen, Norway) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 6-10 November 2023
Report Summary: The formation of bedded barite (BaSO4) deposits in the low-sulfate environments of the early Earth has been a long-standing paradox despite decades of field and geochemical studies. In this project, the team investigated the Si isotope geochemistry of chert (SiO2) dykes and beds found in association with barite to evaluate roles of hydrothermal fluids and seawater during barite formation. A total of 14 chert samples from three localities in the ~3.3 Ga Mapepe and Mendon Formations of the Barberton Greenstone Belt, South Africa were microdrilled and dissolved using sodium hydroxide digestion. Pure silicon fractions were obtained using cation exchange chromatography columns, and analysed for 29Si/28Si and 30Si/28Si isotope ratios by multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) in wet plasma mode using standard-sample bracketing for mass bias correction. Measured silicon isotope ratios (δ30Si) range from 0.27 to 1.29‰. Chert dykes (n = 6) and bedded cherts (n = 3) have similar silicon isotopic compositions, with an average δ30Si value of 0.88‰ for the dykes and 0.80‰ for the bedded cherts. Black chert from the Mendon formation is isotopically distinct (δ30Si = 0.45‰) from the Mapepe Formation cherts. These results tentatively suggest that the chert dykes and bedded cherts associated with barite formed from isotopically-heavy seawater (δ30Si > 0‰), and that the role of high-temperature hydrothermal fluids (δ30Si < 0‰) was limited.
