Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..425r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 425
Mathematics
Logic
6
Carbide, Eh Chondrites, Interstellar Dust, Isotopes
Scientific paper
A light-element stepped-combustion, noble-gas, ion-probe, and SEM study of interstellar SiC from Indarch has been undertaken in order to compare SiC isolated from enstatite chondrites with SiC from the more extensively studied CM2 carbonaceous chondrites. Eighty-five grams of Indarch were etched in HF/HCl, crushed and treated with 9MHF/1MHCl + 1M HCl, Cr(sub)2O(sub)7^2- in H(sub)2SO(sub)4, and then HClO(sub)4, leaving an acid-resistant residue equivalent to 42 ppm of the whole rock. Carbon, nitrogen, and noble-gas data were acquired by stepped combustion and pyrolysis after precombusting the samples to 600 degrees C to oxidize nanometer-sized diamond. The presence of isotopically anomalous SiC in the Indarch residue is indicated by the isotopically heavy CO2 released at high temperature during stepped combustion, with a maximum delta ^13C value of +1420 per mil (^12C/^13C = 36.2), identical to results obtained for typical CM2 samples [1]. In contrast to CM meteorites, however, the peak release of heavy carbon occurs at 1200 degrees C, some 200 degrees C higher than the peak release temperatures of Murchison and Cold Bokkeveld. A similarly high release temperature was seen in the stepped-combustion analysis of the noble gas in the Indarch residue. This suggests a morphological and/or size difference between the SiC present in the two types of meteorites. The nitrogen stepped-combustion profile of the Indarch residue is dominated by the presence of Si(sub)3N(sub)4 of unremarkable isotopic composition (delta ^15N = -56 per mil) that could not be resolved from nitrogen released from SiC. The abundance of SiC in Indarch is estimated, from the stepped combustion data, to be about 1.4 ppm (or 14 ppm SiC in the matrix, not dissimilar to values obtained for CM2 meteorites). A comparison of the noble-gas data from grain-size fractions of Murchison [2] and the bulk Indarch residue data, particularly the Ne-E/Xe-s ratio, suggests that Indarch is enriched in fine-grained SiC. For the ion probe and SEM analysis further purification of the SiC in the residue was necessary, which was achieved by colloidal removal of the diamond fraction and removal of spinel with H(sub)2SO(sub)4 following the procedure of Amari et al. [3]. The sample at this stage consisted mainly of SiC and Si(sub)3N(sub)4. All the SiC in the residue is present in ~20-micrometer clusters of submicrometer-sized grains and, unlike the bulk separates from the CMs, no larger (>1-2-micrometer) single crystals were observed. Ion probe measurements of the clusters indicate that their carbon and silicon isotopic compositions are similar to those of the fine-grained fractions of Murchison [4], consistent with the noble-gas data. This is in contrast to a previous Indarch study [5] that reported only large, isotopically normal grains, but the fine-grained SiC may have been lost [5]. These results imply that, compared to Murchison, the grain-size distribution of SiC from Indarch is skewed to finer grain sizes. The difference in SiC grain size between Indarch and Murchison combined with the dependence of noble-gas composition on grain size suggests that the use of noble gases to determine the SiC content of a meteorite [6] may not always be appropriate. The presence of these aggregates probably explains the elevated combustion temperatures of SiC in Indarch. Judging from their apparent stability during combustion these aggregates could be primary, but, at present, it cannot be excluded that they formed as an artifact. Whether the small grain size of Indarch SiC results from some nebular or parent-body process, or is a presolar phenonemon, is open to debate. Acknowledgments: We thank E. Olsen for the generous donation of the Indarch sample. References: [1] Russell et al. (1991) Meteoritics, 26, 390. [2] Lewis et al. (1990) Nature, 348, 293-298. [3] Amari et al. (1993) GCA, in press. [4] Amari et al. (1991) LPS XXII, 19-20. [5] Stone et al. (1991) EPSL, 107, 570-581. [6] Huss G. (1990) Nature, 347, 159-162.
Arden John W.
O'D. Alexander Conel M.
Ott Ulrich
Pillinger Colin T.
Russell Stuart
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