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Cu and Zn isotope fractionation during extreme chemical weathering

Little, Susan Halsall; Munson, Sophie; Prytulak, Julie; Coles, Barry J; Hammond, Samantha J; Widdowson, Mike

Authors

Susan Halsall Little

Sophie Munson

Julie Prytulak

Barry J Coles

Samantha J Hammond



Abstract

Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales.

Citation

Little, S. H., Munson, S., Prytulak, J., Coles, B. J., Hammond, S. J., & Widdowson, M. (2019). Cu and Zn isotope fractionation during extreme chemical weathering. Geochimica et cosmochimica acta, 263, 85-107. https://doi.org/10.1016/j.gca.2019.07.057

Journal Article Type Article
Acceptance Date Jul 31, 2019
Online Publication Date Aug 7, 2019
Publication Date Oct 15, 2019
Deposit Date Oct 18, 2019
Publicly Available Date Aug 8, 2020
Journal Geochimica et Cosmochimica Acta
Print ISSN 0016-7037
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 263
Pages 85-107
DOI https://doi.org/10.1016/j.gca.2019.07.057
Keywords Cu; Zn; Isotopes; Laterite; Weathering; Organic complexation; Fe oxides
Public URL https://hull-repository.worktribe.com/output/2694050
Publisher URL https://www.sciencedirect.com/science/article/pii/S0016703719304946?via%3Dihub

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