Most Earth-surface calcites precipitate out of isotopic equilibrium

M. Daëron (), R. N. Drysdale, M. Peral, D. Huyghe, D. Blamart, T. B. Coplen, F. Lartaud and G. Zanchetta
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M. Daëron: Université Paris-Saclay
R. N. Drysdale: The University of Melbourne
M. Peral: Université Paris-Saclay
D. Huyghe: Sorbonne Université, CNRS
D. Blamart: Université Paris-Saclay
T. B. Coplen: U.S. Geological Survey
F. Lartaud: Sorbonne Université, CNRS
G. Zanchetta: Università di Pisa

Nature Communications, 2019, vol. 10, issue 1, 1-7

Abstract: Abstract Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that 18O/16O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium 18O/16O fractionation between calcite and water (18αcc/w). Here, we constrain the equilibrium relations linking temperature, 18αcc/w, and clumped isotopes (Δ47) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium 18αcc/w values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect Δ47 in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.

Date: 2019
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DOI: 10.1038/s41467-019-08336-5

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