Chondrite carbon isotopes

A major conclusion from analyses of carbon isotopes in organic compounds in chondrites is that 513C values decrease as the number of carbon atoms in the molecule... 

Halbout J., Mayeda T. K., and Clayton R. N. (1986) Carbon isotopes and light element abundances in carbonaceous chondrites. Earth Planet. Sci. Lett. 80, 1-18. 

Grady M. M. and Pillinger C. T. (1986) Carbon isotope relationships in winonaites and forsterite chondrites. Geochim. Cosmochim. Acta 50, 255-263. 

Benedix S. A., Leshin L. A., Farquhar J., Jackson T. L., and Thiemens M. H. (2000) Carbonates in CM chondrites oxygen isotope geochemistry and impUcations for alteration of the CM parent body. Lunar Planet. Sci. XXXI, 1840. Lunar and Planetary Institute, Houston (CD-ROM). 

Two analytical approaches have been adopted in attempts to obtain stable-isotopic information on insoluble organic matter in carbonaceous chondrites stepped-combustion analysis (e.g., Kerridge, 1983 Swart et al, 1983) and CSIA of pyrolysis products (e.g., Sephton et al, 1998). Stepped-combustion analysis has proved to be more successful in providing information on the major-elemental constituents of chondritic organic matter, i.e., carbon, hydrogen, nitrogen, and oxygen, whereas CSIA has started to yield detailed carbon isotopic and structural information. 

The high carbon contents of ureilites and the distribution of their oxygen-isotopic compositions along the CCAM line suggest that their parent body was originally similar to carbonaceous chondrites. However, no data associate ureilites with a specific carbonaceous chondrite group. The carbon-isotopic compositions of ureilites are distinct from those of any carbonaceous chondrite (Grady et al., 1985). 

Distributions in the solar system. More data on volatiles throughout the solar system are clearly required to confidently describe the volatile acquisition history of the terrestrial planets in the proper context. There are several unknown values for the solar composition, including the nitrogen-and carbon-isotope compositions. The compositions of comets from different orbital distances are needed to assess the extent of radial transport of volatiles late in accretion history. In addition, the causes of carbon- and nitrogen-isotope variations in chondrites must be better understood. While it is clear that the Earth cannot be constructed simply by mixing of different meteorite classes, it is not yet possible to unambiguously extrapolate to the volatile compositions of protoplanetary materials. 

Hoppe P, Amari S, Zinner E, Ireland T, Lewis RS (1994) Carbon, nitrogen, magnesium, silicon and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite. Astrophys J 430 870-890... 

The analysis of fractionation law exponents quantifies the impression from the A -5 plots that aqueous Mg is related to primitive mantle and average crustal Mg by kinetic processes while carbonates precipitated from waters approach isotopic equilibrium with aqueous Mg. In any case, the positive A Mg values of carbonates relative to the primitive chondrite/mantle reservoir and crust is a robust feature of the data and requires a component of kinetic Mg isotope fractionation prior to carbonate formation, as illustrated schematically in Figure 3. 

Robert F, Epstein S (1982) The concentration and isotopic composition of hydrogen, carbon and nitrogen carbonaceous meteorites. Geochim Cosmochim Acta 46 81-95 Robert F, Merlivat L, Javoy M (1978) Water and deuterium content in ordinary chondrites. Mete-oritics 12 349-354... 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

As we will see, some anomalies in the isotopic composition of carbon, hydrogen and oxygen can be explained on the basis of this assumption, and we will start the discussion with the deuterium-rich matter in carbonaceous chondrites. This deuterium-rich matter is essentially present as complex macromolecules 70 73 96 97). The carbon in these samples is essentially normal 76,98). For some polymer-type fractions, the deuterium content is up to 32 times higher than the galactic value (D/H 2 x 10s in the number of atoms per cubic centimeter). High deuterium enrichments are known in interstellar molecules and the mechanism of this enrichment is fully understood. For an excellent review dealing with interstellar chemistry, see the paper by Winnewisser 99) and the previously mentioned book by Duley and Williams 13). 

At least three exotic noble gas components have been detected in carbonaceous chondrites. The first component is so-called carbonaceous chondrite fission xenon, which is enriched in the heavy and light isotopes of this remarkable element with nine stable isotopes. The carrier carbon phase is characterised by a 513C = —38%0, and is called carbon-5. The second component is s-process xenon, which is enriched in even-numbered middle isotopes. The carrier carbon phase is characterised by a 8 = +1100%o, and is called carbon-0. The third component is neon-E(L),... 

Chemical state of carbon in carbonaceous chondrites agrees with that predicted from their formation conditions (indicated by boxes), as inferred from isotopic fractionation of O and C, or abundanc es of volatile metals (Table 1 and Fig. 11 Onuma et al., 1972, 1974 Anders et al., 1976). Cl and C2 chondrites, having formed between 360 and 400 K, contain mainly organic compounds with only traces of carbynes (Whittaker et al., 1980). C3 chondrites contain mainly elemental carbon, which, at least in the case of Allende, is present as carbynes rather than graphite... 

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