¹Shiori Inada, ²Tetsuya Hama, ^1,3Shogo Tachibana
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.07.018]
¹Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
²Komaba Institute for Science and Department of Basic Science, The University of Tokyo, 3-8-1 Komaba, Tokyo 153-8902, Japan
³UTokyo Organization for Planetary and Space Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
Copyright Elsevier

IIsotopic fractionation resulting from kinetic isotope effects (KIEs) in evaporation is a key to investigating high-temperature evaporation events in the early Solar System. The magnitude of the KIEs is represented by the kinetic isotope fractionation factor , which is predicted as  (: the mass ratio of the isotopic evaporated gas species) to a first approximation based on the Hertz-Knudsen equation. However, the experimentally measured  are often closer to 1 than this prediction to various degrees. In this study, we investigated the reason for this observation based on the transition state theory. To evaluate the classical (high-temperature) limit of , which is given by the isotopic ratio of the imaginary frequencies representing the evaporative motion at the transition state, we constructed a simple model for the vibrational normal mode analysis. In this model, we included the effects of the interaction of the evaporating species with the condensed phase surface, as well as the degrees of freedom of atoms in the condensed phase. The present theory clarified the relationship between the magnitude of the evaporative KIEs and the properties of the potential energy surface: the classical limit of  becomes closer to 1 than  due to the effect of the condensed-phase degrees of freedom when there exists a potential energy barrier, which is related to unstable interaction between the evaporating species and the condensed phase surface. This result is consistent with the previous experimental data and provides general insights into classical KIEs in chemical reactions.

¹S.A. Singerling, ¹F.E. Brenker, ¹B. Tkalcec, ²S.S. Russell, ³T.J. Zega, ⁴T.J. McCoy, ^3,5,6H.C. Connolly Jr., ³D.S. Lauretta
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2025.06.028]
¹Schwiete Cosmochemistry Laboratory, Goethe University, Frankfurt, Germany
²Planetary Materials Group, Natural History Museum, London, UK
³Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
⁴Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
⁵Department of Geology, Rowan University, Glassboro, NJ, USA
⁶Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA
Copyright Elsevier

We describe nanoscale observations obtained via transmission electron microscopy of Na,Ca carbonates in OSIRIS-REx samples of asteroid Bennu. Four Na,Ca carbonate grains were observed (including the one briefly described in McCoy and Russell et al., 2025), ranging in size from 140 nm to 2.36 µm. The stoichiometry of the grains and electron diffraction data best match gaylussite (Na2Ca(CO3)2·5H2O) or pirssonite (Na2Ca(CO3)2·2H2O). The grains rapidly amorphized under the electron beam. We also found that the grains are reactive to the terrestrial atmosphere, with their compositions and textures changing over six months of storage in a standard desiccator. NaCl salts grew on the exteriors of the grains, and the compositions of the carbonates became richer in C, F, Cl, and Ca and poorer in O and Na.
Neither gaylussite nor pirssonite have been observed in planetary materials other than samples from Bennu. On Earth, these phases occur in evaporites or shales from alkali lakes and, less commonly, as veins in alkaline igneous rocks. Thermodynamic modeling has shown that both phases require a low-temperature (<55 °C), Na-rich (>140 g/kg Na2CO3) brine, and their presence in the Bennu samples supports a model of salt formation on the parent body during syndepositional back-reaction of a briny fluid (McCoy and Russell et al., 2025). We argue that these minerals have not been previously observed owing either to their rare formation conditions or their susceptibility to degradation from sample preparation and analysis (e.g., electron/ion beam imaging), terrestrial weathering, and/or storage in a terrestrial environment. This study highlights the importance of collecting and carefully preserving pristine samples from planetary bodies.