• A. Garenne
  • P. Beck
  • G. Montes-Hernandez
  • L. Bonal
  • E. Quirico
  • O. Proux
  • J.L. Hazemann
The iron record of asteroidal processes in carbonaceous chondrites
The valence of iron has been used in terrestrial studies to trace the hydrolysis ofprimary silicate rocks. Here, we use a similar approach to characterize the secondaryprocesses, namely thermal metamorphism and aqueous alteration, that have affectedcarbonaceous chondrites. X-ray absorption near-edge structure spectroscopy at the Fe-K-edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. Whileprevious studies have focused on the relative distribution of Fe0with respect to oxidizediron (Feox=Fe2++Fe3+) or the iron distribution in some specific phases (e.g., Urey–Craigdiagram; Urey and Craig 1953), our measurements enable us to assess the fractions of ironin each of its three oxidation states: Fe0,Fe2+, and Fe3+. Among the four carbonaceouschondrites groups studied, a correlation between the iron oxidation index (IOI=[2(Fe2+)+3(Fe3+)]/[FeTOT]) and the hydrogen content is observed. However, within the CMgroup, for which a progressive alteration sequence has been defined, a conversion of Fe3+to Fe2+is observed with increasing degree of aqueous alteration. This reduction of iron canbe explained by an evolution in the mineralogy of the secondary phases. In the case of thefew CM chondrites that experienced some thermal metamorphism, in addition to aqueousalteration, a redox memory of the aqueous alteration is present: a significant fraction ofFe3+is present, together with Fe2+and sometimes Fe0. From our data set, the CRchondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, thethree oxidation states of iron coexist. Even in the least-altered CR chondrites, the fractionof Fe3+can be high (30% for MET 00426). This observation confirms that oxidized ironhas been integrated during formation of fine-grained amorphous material in the matrix (LeGuillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOIof CV chondrites does not reflect the reduced/oxidized classification based on metal andmagnetite proportions, but is strongly correlated with petrographic types. The valence ofiron in CV chondrites therefore appears to be most closely related to thermal history, ratherthan aqueous alteration, even if these processes can occur together (Krot et al. 2004;Brearley and Krot 2013).
carbonaceous chondrites, CM chondrites, CR chondrites, CI chondrites, CV chondrites, spectroscopy, Fe-K-edge XANES
spectral data, material-matter, planetary sciences, instrument-technique
Meteoritics and Planetary Science
2652 - 2665
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