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

International audienceTochilinite/cronstedtite intergrowths are commonly observed as alteration products in CM chondrite matrices but the conditions under which they formed are still largely under-constrained 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 in formed by reaction of matrix amorphous silicates, metal beads, and water at low temperature (50–120°C)