Measurements on Silicon Carbide from Murchison Meteorite: Data for C, N, and SI
Abstract
Interstellar grains are supposed to condense in the mass-losing atmospheres of low-mass red giant stars (M < 3Msolar mass) during their Asymptotic Giant Branch-phase. With respect to the solar system these AGB-stars are expected to produce highly anomalous isotopic compositions of the CNO elements and also of many other light elements [1,2,3]. We have analyzed about 30 single SiC grains (average size 3-5 micrometers) from the Murchison meteorite separate MS1, using a modified CAMECA IMS 3f Ion- microprobe, employing an oxygen primary beam. We have measured the isotopic signatures of C, N as negative secondary ions C- and CN-, and Si as positive secondary ions. According to high-resolution plots, mass 27, which corresponds to CN-, was free from boron contamination. Results: The main argument for an interstellar origin of meteoritic grains is their anomalous isotopic composition. Most of our SiC-grains contained isotopically normal C, but we found three grains with highly anomalous ^12C/^13C ratios: Grain #1 has a ^12C/^13C ratio of 295, grain #2 one of 520. In these two grains only the C isotopes have been analyzed. The ^12C/^13C- ratio of grain #3 is 310 which, together with an N-14/N15-ratio of 300 indicates that this grain might be a 'Y'-grain [4]. The data for the silicon isotopes of most grains fall close to the slope-1.3 line, only two grains are distinct. Their delta values are delta Si^29= (198 +/- 75) permil and delta Si^30=(-170 +/- 20) permil for grain #4 and delta Si^29 =(-955.4 +/- 2.3) permil and delta ^30Si=(-997.7 +/- 0.3) permil for grain #5. The small size of these grains only allowed to measure the Si isotopes. In case of grain 5 the isotopic composition, which is almost pure Si^28, suggests an origin in a supernova, or, according to Brown and Clayton, that the Si might be produced by Mg-burning in the He-burning shell of an AGB-star [5]. References : [1] Gallino R. et al. (1990) Nature, 348, 298. [2] Zinner E. et al. (1989) GCA, 53, 3273-3290. [3] Ott, U. (1993) Nature, 364, 25. [4] Amari S. et al. (1992) LPS XIII, 27. [5] Brown L. E. and Clayton D. D.(1992) Astrophys. J., 392, L79.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29R.550W
- Keywords:
-
- Chemical Composition;
- Interstellar Matter;
- Meteoritic Composition;
- Murchison Meteorite;
- Silicon Carbides;
- Anomalies;
- Asymptotic Giant Branch Stars;
- Carbon Isotopes;
- Ion Probes;
- Nitrogen Isotopes;
- Silicon Isotopes;
- Lunar and Planetary Exploration;
- AGB-STAR; ION PROBE; ISOTOPIC ANOMALIES; MG-BURNING; NITROGEN; SILICON CARBIDE; SUPERNOVA