Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.318a&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 318
Other
4
Carbonaceous Chondrites, Cr Chondrites, Isotopic Variation, Organic Compounds, Primitive Meteorites
Scientific paper
The CR chondrites are now well established as an entity to be considered alongside other carbonaceous chondrites and can no longer be classified as a subset of another group. The isotopic composition of nitrogen and hydrogen in these meteorites is diagnostic of the group, each being highly enriched in the heavy isotope. The source and history of these isotopic signatures can be explained by the survival of presolar organic materials in these meteorites. Astronomical Observations of organic material in interstellar clouds imply D/H ratios in the region of 1 (ca. 10^6%o). The much lower observed meteoritic deuterium overabundance (up to 5750%o [1]) is generally attributed to the survival of small quantities of presolar organic material, but in a form much diluted by solar system-produced material. The concentration of deuterium observed in the interstellar cloud organics is produced by low temperature ion-molecule reactions and a similar, but smaller scale, phenomenon in ^15N distribution has been postulated by some authors [2] with some astronomical measurements appearing to support this [3,4]. From chemical considerations it is not feasible for the carbon to produce such isotopic signatures from interstellar chemical reactions. While deuterium enrichments accompanied by ^15N enrichments have been found in some meteorites (e.g. Semarkona Delta D = +5740%o [5], Delta^15N = +65%o) the effects in nitrogen are generally small. The CR chondrites, however, show an enrichment in both deuterium (up to +1300%o for whole rock values) and a substantial enrichment in ^15N (up to +185%o) and some authors have postulated a possible correlation of ^15N and deuterium ennchments [e.g., 6]. New data obtained by stepped combustion support this hypothesis (see Fig. 1), and also show that the material is carbonaceous and burns at a low (<500 degrees C) temperature suggesting an organic nature. The stepped combustion allows the nitrogen isotopic composition of the organic material to be determined, which is generally heavier than the whole rock value. A plot of the organic Delta ^15N vs. Delta D also show a correlation, but the effect of the non-organic material is generally small. Extrapolation of the correlation line shows an intercept with the values obtained for Orgueil organic material, suggesting the presence of some similar presolar organics in the CI chondrites. The smaller isotopic enrichments in Orgueil are due to the presence of more solar system derived organic material masking the isotopic signature. The preservation of high concentrations of presolar organic material, as well as the high concentrations of other, more refractory presolar materials, is difficult to reconcile with the extensive hydrous alteration but the highly reduced nature of the meteorite may have helped the survival of these materials. References: [1] Robert et al. (1987) GCA, 51, 1807. [2] Adams and Smith (1981) Astrophys. J. Lett., 247, L123. [3] Audouze et al. (1975) Astron. Astrophys., 43, 71. [4] Hermsen et al. (1985) Astron. Astroph., 146, 134. [5] McNaughton et al. (1982) Proc. LPSC 13th, in JGR, 87, A297. [6] Kerridge (1985) GCA, 49, 1707. Fig. 1 appears here in the hard copy.
Ash Richard D.
Morse Andrew D.
Pillinger Colin T.
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