Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.413n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 413
Other
6
Interstellar Dust, Ion Imaging, Isotopic Anomalies, Murchison, Silicon Carbide
Scientific paper
Previous ion microprobe analyses of individual interstellar SiC grains from primitive meteorites have revealed a small fraction (<=1%) of grains (X grains) whose isotopic compositions differ markedly from those of the majority of grains [1,2]. "Mainstream" SiC is characterized by enriched ^13C, ^14N, ^29Si, ^30Si, ^46Ti, ^47Ti, ^49Ti, and ^50Ti relative to solar composition, and is thought to have formed in AGB stars [3]. X grains, on the other hand, have large depletions in ^29Si and ^30Si, as well as light carbon, heavy N, enriched ^49Ti, and (^26Al/^27Al)(sub)o ratios some 2-3 orders of magnitude higher than typical SiC. Amari et al. have argued that the most likely sources for X grains are supernovae [1]. To better characterize these rare grains and to delineate their astrophysical sources, we have used an ion imaging system [4] to more efficiently locate exotic SiC grains. Images in ^28Si and ^30Si were digitized by a CCD camera and 30Si/28Si ratios of individual grains were determined by image processing. Over a period of three days, we mapped the ^30Si/^28Si ratios in some 1250 2- 4-micrometer grains from the Murchison separate KJG, and located nine grains with large depletions in ^30Si. Subsequent measurements at high mass resolution of Si, C, and N isotopes in these grains confirmed the ^30Si depletions and showed isotopic signatures similar to those of the previously studied X grains. Measurements of Mg, Ca and Ti isotopes are planned for these grains. Silicon isotopic data for both the nine new grains and seven previously studied X grains are shown in Fig. 1; these grains all lie deep inside the ^28Si-enriched quadrant. Seven of the new grains lie close to a line defined by most previous X grains, but two, 47-3 and 103-1, lie near X2, which falls on a mixing line of solar composition with pure ^28Si. In addition to its distinctive Si isotopic composition, grain X2 shows a large excess of ^44Ca (300%) not seen in other grains. Additional Ca isotopic measurements on the new grains should help decide whether grains like X2 represent a distinct subpopulation. Eight of the new grains show light carbon (^12C/^13C= 210-1272) and heavy nitrogen (^14N/^15N = 55-125), similar to the other X grains, although the range of ^15N deviations is not as large as previously seen. One unusual grain, 103-1, is highly enriched in ^13C (^12C/^13C= 42.6), unlike all the other X grains, whereas its ^14N/^15N ratio is very similar to that of the other grains. A few SiC grains with heavy carbon and light Si have previously been found, although they are much less enriched in ^28Si than the X grains [2]. Whether grain 103-1 is related to these grains is an open question. References: [1] Amari et al. (1992) Astrophys. J. Lett., 394, L43-L46. [2] Alexander C. M. O'D. (1993) GCA, in press. [3] Hoppe P. et al. (1993) Astrophys. J., submitted. [4] Nittler et al. (1993) LPS XXIV, 1087. Fig. 1 appears here in the hard copy.
Amari Sachiko
Lewis Reed S.
Nittler Larry R.
Walker Rober M.
Zinner Ernst K.
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