  The valence of iron has been used in terrestrial studies to trace the
hydrolysis of primary silicate rocks. Here, we use a similar approach to
characterize the secondary processes, namely thermal metamorphism and aqueous
alteration, that have affected carbonaceous 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. Among the four carbonaceous chondrites
groups studied, a correlation between the iron oxidation index (IOI = [2
((Fe2+) + 3(Fe3+))/FeTOT) and the hydrogen content is observed. However, within
the CM group, 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 can be explained by an evolution in the
mineralogy of the secondary phases. In the case of the few CM chondrites that
experienced some thermal metamorphism, in addition to aqueous alteration, a
redox memory of the aqueous alteration is present: a significant fraction of 3+
2+ 0 Fe is present, together with Fe and sometimes Fe. From our data set, the
CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR
chondrites, the three oxidation states of iron coexist. Even in the
least-altered CR chondrites, the fraction of Fe3+ can be high (30% for MET
00426). This observation confirms that oxidized iron has been integrated during
formation of fine-grained amorphous material in the matrix. Last, the IOI of CV
chondrites does not reflect the reduced/oxidized classification based on metal
and magnetite proportions, but is strongly correlated with petrographic types.
The valence of iron in CV chondrites therefore appears to be most closely
related to thermal history, rather than aqueous alteration, even if these
processes can occur together .
