Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments

1Lionel G. Vacher,2Laurent Truche,1François Faure,1Laurent Tissandier,3Régine Mosser‐Ruck,1Yves Marrocchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13317]
1CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐les‐Nancy, F‐54501 France
2ISTerre, UMR 5275, CNRS, Université Grenoble Alpes, 1381 rue de la Piscine, BP53 38041 Grenoble, CEDEX 9, France
3GeoRessources, UMR 7359, CNRS, Université de Lorraine, Campus Aiguillettes, 54506 Vandoeuvre‐lès‐Nancy, France
Published by arrangement with John Wiley & Sons

Tochilinite/cronstedtite intergrowths are commonly observed as alteration products in CM chondrite matrices, but the conditions under which they formed are still largely underconstrained due to their scarcity in terrestrial environments. Here, we report low temperature (80 °C) anoxic hydrothermal experiments using starting assemblages similar to the constituents of the matrices of the most pristine CM chondrite and S‐rich and S‐free fluids. Cronstedtite crystals formed only in S‐free experiments under circumneutral conditions with the highest Fe/Si ratios. Fe‐rich tochilinite with chemical and structural characteristics similar to chondritic tochilinite was observed in S‐bearing experiments. We observed a positive correlation between the Mg content in the hydroxide layer of synthetic tochilinite and temperature, suggesting that the composition of tochilinite is a proxy for the alteration temperature in CM chondrites. Using this relation, we estimate the mean precipitation temperatures of tochilinite to be 120–160 °C for CM chondrites. Given the different temperature ranges of tochilinite and cronstedtite in our experiments, we propose that Fe‐rich tochilinite crystals resulted from the alteration of metal beads under S‐bearing alkaline conditions at T = 120–160 °C followed by cronstedtite crystals formed by the reaction of matrix amorphous silicates, metal beads, and water at a low temperature (50–120 °C).

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