Carbonate abundances and isotopic compositions in chondrites

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Abstract

Abstract We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba ( CV 3), and Semarkona ( LL 3.0). For the unheated CM s, the total ranges of carbonate isotopic compositions are δ 13 C ≈ 25–75‰ and δ 18 O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ 18 O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH 4 (δ 13 C ≈ −33‰ or −20‰ for CO ‐ or CH 4 ‐dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ 18 O ≈ −5.5‰, and δ 13 C ≈ −31‰ or −17‰ for CO ‐ or CH 4 ‐dominated systems, respectively) can explain the carbonate compositions of the CI s, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CM s. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ 13 C ≈ 65–80‰) and less altered samples (δ 13 C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ 18 O ≈ −9.25‰, and δ 13 C ≈ −21‰ or −8‰ for CO ‐ or CH 4 ‐dominated systems, respectively) if the less altered CR s had higher mole fractions of CO 2 in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO 2 ‐rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO 2 and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO ‐dominated fluids to have formed in their parent bodies. If CH 4 was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO 2 when oxidation of metal by water created high partial pressures of H 2 . The fact that the chondrite carbonate C/H 2 O mole ratios are of the order predicted for CO / CO 2 ‐H 2 O ices that experienced temperatures of >50–100 K suggests that the chondrites formed at radial distances of <4–15 AU.

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