Russell N Drysdale Marion Peral Damien Huyghe Dominique Blamart Tyler B. Coplen Franck Lartaud Giovanni Zanchetta Mathieu Daeron 2019 <p><span>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&nbsp;</span>¹⁸<span>O/</span>¹⁶<span>O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium&nbsp;</span>¹⁸<span>O/</span>¹⁶<span>O fractionation between calcite and water (</span>¹⁸<i>α</i>_cc/w<span>). Here, we constrain the equilibrium relations linking temperature,&nbsp;</span>¹⁸<i>α</i>_cc/w<span>, and clumped isotopes (</span><i>Δ</i>₄₇<span>) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium&nbsp;</span>¹⁸<i>α</i>_cc/w<span>&nbsp;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&nbsp;</span><i>Δ</i>₄₇<span>&nbsp;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.</span></p> application/pdf 10.1038/s41467-019-08336-5 en Nature Most Earth-surface calcites precipitate out of isotopic equilibrium article