^1,2Damien Daval,³Gaël Choblet,³Christophe Sotin,⁴François Guyot
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2021JE006995]
¹Université de Strasbourg / CNRS / ENGEES – Institut Terre et Environnement de Strasbourg, UMR, 7063 Strasbourg, France
²Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, Intitut des Sciences de la Terre, Grenoble, France
³Université de Nantes / CNRS – Laboratoire de Planétologie et Géodynamique, UMR, 6112 Nantes, France
⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Museum National d’Histoire Naturelle, UMR, Sorbonne-Université / CNRS, 7590 Paris, France
Published by arrangement with John Wiley & Sons

The Cassini spacecraft demonstrated that Saturn’s small moon Enceladus may harbor hydrothermal activity. In particular, molecular hydrogen production could result from water-rock interactions in a tidally-heated, water-filled porous rocky core. The lifetime of such reactions is key to assess the habitability potential of Enceladus and to constrain plausible durations of the active stage in a context where the evolution of the moon is debated. Although it has recently been suggested that the serpentinization timescale does not exceed a few hundred million years, this estimation was based on assumptions regarding silicate dissolution kinetics that are prone to overestimate the actual reactivity of primary silicates. Here, we investigated plausible rate-limiting mechanisms governing fluid-rock interactions that could delay the completion of Enceladus’ core serpentinization. In particular, we considered the impact of (i) various secondary mineral assemblages on the Gibbs free energy of Fe-bearing silicate dissolution and associated dissolution rates; (ii) rate-laws alternative to the transition state theory; (iii) diffusion in nanoporous secondary assemblages; (iv) slow water supply. Overall, our results confirm that serpentinization timescales never exceed 500 Myr, and indicate that fluid flow ultimately sets the tempo for serpentinization. Only unreasonable grain sizes in Enceladus’ core (> 1m) or unexpectedly low diffusivity of secondary coatings covering primary silicates would be consistent with serpentinization durations of several billion years. We thus suggest that either the hydrothermal activity has developed recently on Enceladus, or alternative processes (pyrolysis of insoluble organic matter, microbial activity) must be tested to explain the observed H2 flux in Enceladus’ plume.

¹M. A. Torcivia,¹C. R. Neal
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006799]
¹University of Notre Dame
Published by arrangement with John Wiley & Sons

The ferroan anorthosite suite (FAS) represents the only direct sampling of the lunar magma ocean (LMO) and potentially contains information on the earliest history of the Moon. Apollo 16 FAS sample 60025 is extremely important for understanding early lunar evolution, but unraveling this information is complicated. For example, 60025 has two distinct Sm-Nd crystallization ages that in themselves encapsulate the complicated history of this sample, along with the cataclastic and heterogeneous textures exhibited. Here we present new in-situ major and trace element plagioclase and pyroxene data gathered from 5 thin sections of 60025 that highlight such complexities. Trace element data are used to derive equilibrium liquids and while many of the minerals analyzed here are consistent with derivation from the LMO, there are also a significant number of plagioclase and pyroxene crystals that crystallized from magmas inconsistent with current models of LMO evolution. Integration of Sm-Nd isotopic data with the elemental data reported here indicated a non-chondritic LMO is possible and we confirm that 60025 is a polymict lunar breccia containing differently sourced material.