Incipient aqueous alteration recorded by matrix metal in the most pristine CO chondrite dominion range 08006
Shaofan Che, Adrian J. Brearley
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 29 April 2026
“Evidence of low-temperature aqueous alteration is extremely limited in CO (Ornans-type) chondrites: only a minor amount of phyllosilicates have been reported in a few members of the CO chondrite group. This observation seems to be at odds with the fact that CO chondrites of higher petrologic types (≥CO3.2) are widely affected by metasomatism. Deciphering the timing and role of fluid activities is thus a prerequisite to a comprehensive understanding of the secondary alteration history of CO chondrites. In this study, we report the widespread occurrence of submicron-sized, segmented metal assemblages in the matrix of Dominion Range (DOM) 08006, the most pristine CO chondrite known that is classified as type 3.00. The Fe,Ni metal grains are kamacite (α-Fe, body-centered cubic, bcc) that are segmented by lamellae of C-bearing material that define crystallographically controlled alteration interfaces parallel to {110} planes of the kamacite lattice. The segmented metal subgrains are rimmed by two nanometer-wide layers that are composed of an inner, Ni-enriched layer and an outer, amorphous Fe-O layer. Transmission electron microscopic data for the C-bearing regions are consistent with amorphous carbon which encloses nanoparticles of ferrihydrite.
The spatial association between metal and C-bearing regions in the segmented metal assemblages suggests that the precursor phase of the amorphous carbon in the C-bearing regions was likely mobilized during aqueous alteration of the metal on the parent body of DOM 08006. The association of ferrihydrite nanocrystals with amorphous carbon in DOM 08006 likely reflects a coprecipitation process analogous to that observed in terrestrial soils, where oxidation of Fe,Ni metal under near-neutral to slightly acidic aqueous conditions produces Fe3+-bearing hydroxides that complex with dissolved organic matter. This aqueous fluid was most likely generated by melting of water ice that accreted together with organic matter as composite particles. Therefore, the matrix metal in DOM 08006 has recorded incipient aqueous alteration on the CO3 chondrite parent body prior to the onset of thermal metamorphism. However, this aqueous alteration event only affected DOM 08006 to a subtle extent, probably due to the minor amount of fluid available for alteration. Our study of DOM 08006, combined with observations from previous studies of CO chondrites, further suggests that a small amount of low-temperature aqueous fluid could be present on the CO chondrite parent body, resulting in limited effects of aqueous alteration”
