The Iron speciation and valence states in hydrous carbonaceous chondrites

Iron (Fe) is a useful tracer of geo- and cosmo-chemical processes and their associated physicochemical conditions. The Fe redox (reduction-oxidation) states of chondrites have been examined to infer processes that occurred in the early solar system, including pre-accretionary and post-accretionary stages. However, investigations of hydrous carbonaceous chondrites and the interpretation of their Fe mineralogy have remained very limited. In this study, we have characterized the Fe speciation (i.e., distribution of Fe in mineral phases) and the Fe valence states of clay minerals in various hydrous carbonaceous chondrites, including CI, CM, CR, and Tagish Lake-like type meteorites, as well as Ryugu and Bennu samples, combining synchrotron Mössbauer spectroscopy and X-ray diffraction.
In contrast to the classical view that Fe in hydrous carbonaceous chondrites is accommodated only by oxides or silicates, we identify substantial amounts of Fe sulfides and Fe metal in fresh CI and less altered CM samples, respectively. In addition, the metal-bearing CM samples have Fe3+-rich poorly-crystallized phases, and the Fe3+/(Fe3++Fe2+) ratio in clay minerals decreases with increasing alteration intensity.
Our results indicate that hydrous carbonaceous chondritic materials formed and evolved under less oxidizing conditions than previously thought. Moreover, our findings on undifferentiated meteorites coincide with recent studies of differentiated meteorites, together suggesting that redox conditions (mainly as indexed by oxygen fugacity) do not differ between the inner and outer parts of the early solar system.