Investigation of organo-carbonate associations in carbonaceous chondrites by Raman spectroscopy

Carbonates record information regarding the timing, nature and conditions of the fluids circulating through asteroid parent bodies during aqueous alteration events. Determining carbonate abundances and their relationships with organic matter improves our understanding of the genesis of major carbonaceous components in chondritic materials. In this study, five CM2 carbonaceous chondrites (CM2.2 Nogoya, CM2.3 Jbilet Winselwan, CM2.5 Murchison, CM2 Santa Cruz, and CM2TII Wisconsin Range 91600) were studied with Raman spectroscopy. Carbonates were identified in these meteorite samples by the distinctive Raman band in the similar to 1100 cm(-1) region, representing the symmetric stretching vibration mode (nu(1)) of the (CO3)(2-) anion. Carbonates identified in the meteorite samples are all calcite, with the exception of a single dolomite grain in Nogoya. The v(1) positions of the CM calcites are 2-3 cm(-1) higher than in pure calcite, which suggests that they contain significant impurity cations. Typical graphitic first-order D and G bands were identified in the meteorite matrix as well as in similar to 25% of the analyzed carbonate grains. From the Raman results, we postulate that the carbonates might not have formed under equilibrium conditions from a single fluid. The first generation of carbonate is interpreted to have formed from highly oxidized fluids that led to the oxidation of organic matter (OM) and produced carbonates that are OM-barren. The second generation of carbonate was formed from a more evolved aqueous fluid with the presence of OM. The Raman parameters of the organics in carbonates clearly deviate from the matrix OM which suggests that the carbonate organics contain very different carbonaceous components that are distinct from the typical amorphous OM of the CM matrix. The occurrence of different generations of carbonate in close proximity may be partly responsible for the wide range in estimated ages of carbonates in carbonaceous chondrites reported in previous studies. (C) 2016 Published by Elsevier Ltd.