What do fluid lithium isotopes record: Insights from long-term water-rock interaction experiments

Liu, Xianyi, Wilson, David J., Krause, Alexander J. ORCID: https://orcid.org/0000-0002-9771-8101, Dahl, Tais W., Sarkar, Sneha Ray, Fraser, Wesley T., Liu, Chunyao and Pogge von Strandmann, Philip A.E. 2027. What do fluid lithium isotopes record: Insights from long-term water-rock interaction experiments. Chemical Geology 727 , 123790. 10.1016/j.chemgeo.2026.123790

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Abstract

Lithium isotopes are widely used to trace silicate weathering and its role in the long-term carbon cycle, yet whether fluid δ7Li values track weathering congruency, fluid residence time, or the reaction rate of solids remains debated. Here, we investigate this question using year-long water-rock interaction experiments with a range of lithologies (including gneiss and granite), and two starting-fluid compositions, combined with quantitative Li mass-balance modelling. Across all the experiments, dissolved δ7Li values increased rapidly within days to weeks, reflecting preferential removal of 6Li into rapidly forming Al-bearing secondary phases and/or adsorption sites. The mass-balance results demonstrate that fluid δ7Li values do not uniquely record weathering congruency: both monotonic and hump-shaped relationships emerge from the evolving imbalance between Li supply and removal, without requiring secondary-mineral dissolution. Moreover, δ7Li values show only a subtle relationship with the solid reaction rate. Fluid residence time primarily determines the system's progression towards a quasi-steady-state for Li, beyond which dissolved δ7Li values become buffered and largely insensitive to further increases in fluid residence time. These results suggest that the δ7Li evolution reflects combined effects from weathering congruency, fluid residence time, and solid reaction rate, and thus cannot be a unique tracer for a single factor. In contrast, we propose that Li concentrations at quasi-steady-state retain information on the integrated Li release rate of the weathering solids, potentially providing complementary constraints on rock reactivity and weatherability. Overall, our results provide a new explanation for the restricted range of δ7Li values observed in large rivers, and an updated conceptual framework for interpreting coupled Li concentration-isotope systematics in modern and ancient silicate weathering systems.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Earth and Environmental Sciences Schools > Physical, Chemical & Environmental Sciences
Publisher:	Elsevier
ISSN:	0009-2541
Date of First Compliant Deposit:	3 November 2026
Date of Acceptance:	25 October 2026
Last Modified:	03 Nov 2026 10:30
URI:	https://orca.cardiff.ac.uk/id/eprint/190611

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