Barium isotope fractionation during the experimental transformation of aragonite to witherite and of gypsum to barite, and the effect of ion (de)solvation

ABSTRACT

In this study, we present the experimental results for stable barium (Ba) isotope fractionation (¹³⁷Ba/¹³⁴Ba) during the transformation of aragonite (CaCO₃) and gypsum (CaSO₄·2H₂O) in Ba-bearing aqueous solution to witherite (BaCO₃) and barite (BaSO₄), respectively. The process was studied at three temperatures between 4 and 60 °C. In all cases, the transformation leads to a relative enrichment of the lighter ¹³⁴Ba isotope in the solid compared to the aqueous solution, with ^137/134Ba enrichment factors between –0.11 and −0.17 ‰ for BaCO₃, and –0.21 and –0.26 ‰ for BaSO₄. The corresponding mass-dependent ^138/134Ba enrichment factors are −0.15 to –0.23 ‰ for BaCO₃, and –0.28 to –0.35 ‰ for BaSO₄. The magnitude of isotope fractionation is within the range of recent reports for witherite and barite formation, as well as trace Ba incorporation into orthorhombic aragonite, and no substantial impact of temperature can be found between 4 and 80 °C. In previous studies, ion (de)solvation has been suggested to impact both the crystallization process of Ba-bearing solids and associated Ba isotope fractionation. Precipitation experiments of BaSO₄ and BaCO₃ using an methanol-containing aqueous solution indicate only a minor effect of ion and crystal surface (de)solvation on the overall Ba isotope fractionation process.

KEYWORDS:

• Aragonite
• barium carbonate
• barium isotopes
• barium sulfate
• gypsum
• ion (de)solvation
• isotope fractionation
• isotope geochemistry
• precipitation
• transformation experiment

Acknowledgements

MEB wishes to further acknowledge contributions to the atmosphere during manuscript preparation from E. Berg and M.-U. Kling, and dedicates the publication to the memory of L.C. Bull-Berg, who recently passed away. We also wish thank R. Bahlo, O. Dellwig and D. Benesch (IOW) for SEM–EDX and ICP-OES support, respectively. N. Pierau, born Neubert, was supported by a stipend from Leibniz IOW. We appreciate the constructive comments by Z. Cao and an anonymous reviewer that helped to improve the manuscript and G. Strauch for his editorial support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Support of this study came from the Leibniz IOW, the Swiss National Science Foundation (grants 200021-107505 and 20020-140618), and the University of Fribourg, Switzerland, which is gratefully acknowledged.