Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..316a&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 316
Other
4
Graphite Inclusion, Interstellar Dust, Ion Probe, Isotopic Anomalies, Murchison
Scientific paper
We report C and N isotopic ratio measurements of 85 graphite grains from the Murchison density fraction KE5 (1.72-1.89g/cm^3, <3 micrometers). In this fraction, one third of the carbon grains give lower C count rates in SEM-EDX analysis, indicating low density. This and their shapes suggest that these grains are not graphitic. All these low density grains have normal C but some have heavy N (^14N/^15N down to 147). Two thirds (59) of all grains give higher X-ray C counts. Among them, 14 grains are blocky, most have normal C. However, 2 blocky grains have heavy C (^12C/^13C = 5.5 and 7.7). The rest of the dense grains can be classified into 3 subgroups according to their morphology. Cauliflower-type grains (which appear to be aggregates of small scales) and onion-type grains (which have smooth surfaces with a shell-like structure) are also common in other graphite density fractions (KFA1-KFC1 [1]). However, there are grains, rare in other graphite fractions, which display features of both types; they have smooth surfaces, but the fine aggregate structure is still apparent (Caulionion-type). As shown in Fig. 1, grains with different morphology have different distributions of their C isotopic ratios. As in the two heavy density fractions KFB1 and KFC1 [2], C isotopic ratios depend on morphology. We measured the ^18O/^16O ratios of 9 grains; 3 have ^18O excesses with ^18O/^16O ratios as low as 68. As previously observed in graphite density fractions KE1 and KFA1 [3], these 3 grains have ^15N excesses. One of them has delta^29Si =- 299 per mil, and delta^30Si = -543 per mil. It has light C (^12C/^13C = 276) and heavy N (^14N/^15N = 230). These C, N, and Si isotopic compositions are the signature of SiC Grains X [4]. Amari et al. [4] have proposed a supernova origin for Grains X. AGB and Wolf- Rayet stars fail to explain the Si isotopic ratios and grains from novae should have much lower ^12C/^13C and ^14N/^15N ratios. In preSNII stars, the He-shell is the only region with C > O, required for graphite formation. While Si in the He-shell is expected to be enriched in the neutron-rich isotopes, a small admixture with material having experienced O burning could explain a ^28Si enrichment. In the He-shell, ^14N is considered to be destroyed more rapidly than ^15N and the ^14N/^15N ratio may thus reach higher than solar values. Furthermore, ^18O is expected to be enriched relative to solar (^18O/^16O = 0.2) [5,6], while ^17O is moderately depleted (^17O/^16O = 2x10^-4). However, to explain the ^12C/^13C ratio of the ^28Si-rich grain, mixing with material from the H-shell is required. Mixing is also indicated by the range of the ^18O excesses in ^18O-rich grains. REFERENCES: [1] Amari S. et al. (1993) GCA, in press. [2] Amari S. et al. (1993) Proc. 18th Symp. on Ant. Met., in press. [3] Hoppe P. et al. (1992) Meteoritics, 27, 235. [4] Amari S. et al. (1992) Ap. J., 394, L43-L46. [5] Weaver T. A. and Woosley S. E. (1993) Phys. Rep., submitted. [6] Hashimoto M. et al. (1993) In Nuclei in the Cosmos, 587-594.
Amari Sachiko
Lewis Reed S.
Zinner Emst
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