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

Abstract

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).

Apatite halogen and hydrogen isotope constraints on the conditions of hydrothermal alteration in carbonaceous chondrites

“…Unfortunately, however, there is currently no experimental or empirical data linking apatite-water H isotope fractionation and temperature. Qualitatively, the large ΔD Apatite-Water values above 100‰ for the Bench Crater and Boriskino carbonaceous chondrites suggest apatite formation at low temperatures, which is in line with temperature estimates for hydrothermal alteration in CI and CM chondrite parent bodies below approximately 150°C (Clayton and Mayeda 1984;Benedix et al 2003;Guo and Eiler 2007;Alexander et al 2015;Verdier-Paoletti et al 2017;Vacher et al 2019aVacher et al , 2019b.…”

“…Among meteorites, water-rich CI and carbonaceous chondrites are of fundamental interest as they might have contributed to the delivery of water to the Earth and other inner solar system rocky bodies (Dauphas and Morbidelli 2014;Sarafian et al 2014;Barnes et al 2016a;Raymond and Izidoro 2017;Vacher et al 2020). Hydrogen and oxygen isotopic analyses are commonly used at both the bulk rock and mineral scales for quantifying (1) the origin and abundance of water and volatile elements in the asteroid-forming region(s) (e.g., Alexander et al 2012Alexander et al , 2018Vacher et al 2016Vacher et al , 2020Marrocchi et al 2018;Piani and Marrocchi 2018;Piani et al , 2020, and (2) the physicochemical and thermal conditions of fluid/rock interactions in chondritic parent bodies (e.g., Mayeda 1984, 1999;Benedix et al 2003;Fujiya et al 2015Fujiya et al , 2019Pignatelli et al 2016Pignatelli et al , 2017Vacher et al 2017Vacher et al , 2018Vacher et al , 2019aVacher et al , 2019bVerdier-Paoletti et al 2017, 2019Telus et al 2019;Piralla et al 2020). Despite being powerful, these isotopic fingerprinting approaches bring only few constraints on the nature of the alteration fluids, and the source and quantity of dissolved volatile species (e.g., molecules comprising F, Cl, S, C, or N).…”

Abundant extraterrestrial amino acids in the primitive CM carbonaceous chondrite Asuka 12236

“…2017; Vacher et al. 2019). During this alteration phase, low temperature chemical oxidation reactions could have resulted in the removal of aliphatic moieties from both soluble and insoluble macromolecular organic matter (Sephton et al.…”

An evolutionary system of mineralogy, Part V: Aqueous and thermal alteration of planetesimals (~4565 to 4550 Ma)

“…A variety of fine-grained layered phases in altered meteorites, once termed "poorly characterized phases" or PCPs (e.g., Barber et al 1983), are more properly referred to as tochilinite-cronstedtite intergrowths or "TCI" (e.g., Vacher et al 2019). TCIs occur in multiple morphotypes, including needle-like enrolled layers and as undulating layers with a dominant 10.8-Å spacing (Barber et al 1983).…”

“…Tochilinite-Cronstedtite Intergrowths (TCIs): A variety of matrix phases, notably in CM chondrites, are poorly crystallized to amorphous and thus difficult to catalog with IMA protocols. Fuchs et al (1973) termed a suite of these materials that incorporate Ni, S, Mg, Fe, and Si as "poorly characterized phases" or "PCPs," though that designation has been replaced by TCIs (e.g., Vacher et al 2019). Bunch and Chang (1980) characterized three compositional types (originally called PCP I, II, and III), which have subsequently been shown to represent a continuum of complexly interlayered mixtures of cronstedtite, an Fe-rich serpentine, and tochilinite, which is itself an ordered sequence of mackinawite-and brucite-type layers (Mackinnon and Zolensky 1984;Nakamura and Nakamuta 1996).…”