S and O chemical analysis of aqueously altered carbonaceous chondrites IOM

CI, CM, CR and ungrouped C2 carbonaceous chondrites contain a few percents of insoluble organic matter (IOM), which could have a presolar origin and provide clues on carbon chemistry in interstellar medium of protostellar environments. However, these chondrites have experienced complex hydrothermal processes in their parent bodies [1-3]. A major issue is the understanding of chemical changes in composition and structure of IOM due to aqueous alteration. Chemical variations have been reported among various chondrites, and so far interpreted as the consequence of oxidation processes by some authors [4; 5]. Variations among the precursors accreted can also be suspected [6]. We measured by micro-infrared spectroscopy (at LPG, Grenoble) and sulfur XANES at the K-edge (at ESRF, ID21, Grenoble) a series of IOMs extracted from CI1 (Ivuna, Orgueil, Alais), CM2 (Murchison, Cold Bokkeveld, QUE99355, QUE97990, Mighei and Murray), CR2 (Renazzo) and C2 ungrouped (Tagish Lake) chondrites. Only CIs and Cold Bokkeveld have strong carboxylic IR bands and oxidised organic sulfur XANES features. Neither XANES and IR show major variability within one group. These results evidence systematic differences between C1 and other C2 chondrites. Cold Bokkeveld appears as a peculiar object, and should be considered as an anomalous CM2. The lack of organic oxidation trend going with alteration gradient makes us say : i) hydrothermalism was very stronger on C1 parent body than on C2's and ii) oxidation of different precursors of C1 and C2 may have an effect on the IOM.