Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.197a&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 197
Other
5
Scientific paper
We have measured selected trace elements in 60 individual SiC grains from the Murchison separate KJH [average grain size 4.57 micrometers (Amari et al., 1982)] whose C, N, and Si isotopic compositions had previously been determined. We also measured aggregates of the fine-grained (average size 0.49 micrometers) separate KJB whose C, N, Mg, Si, Ca, Ti, Ba, Nd, Sm, and noble gas isotopic compositions had been analyzed earlier (Amari et al., 1991a, 1991b; Zinner et al., 1991; Lewis et al., 1992). The Si and CI normalized elemental abundance pattern of KJB (Fig. 1a) appears to be affected by chemical and nucleosynthetic effects. Mg and Fe are too volatile to be incorporated into SiC. Ti and V, expected to be at least as refractory as Si (Lattimer et al., 1978; Fegley, 1991), have approximately the same abundance as Si. The heavy elements from Y on (Sr is depleted because of volatility) are overabundant by factors 20-30. In C-stars a large fraction of the Si is expected to condense as SiC (Lattimer et al., 1978); thus, this overabundance, coupled with the identification of Ba and Nd as almost pure s-process, confirms the identification of AGB stars as the most likely source of fine-grained SiC (Lewis et al., 1990; Gallino et al., 1990) as such stars are expected to produce large overabundances of heavy elements (Gallino et al., 1990; Kappler et al., 1990). Single KJH grains have large variations in their trace elements (Figs. 1b and 1c). Some patterns are similar to that of KJB (Fig. 1b). Sr and Ba depletions relative to their respective neighbors are correlated but variable, indicating different SiC condensation temperatures (note the Al depletion in grain KJH10-551). Some grains (KJH5-042 and KJH8-241) lack any heavy (>=Y) element enrichments. They either formed in AGB stars before the dredge-up of s-process material from the He-burning shell or come from other types of stars (e.g., WR-stars). Many grains show large depletions in Al, Ti, and V relative to Si (Fig. 1c). Since these depletions seem to be unrelated to Sr depletions (cf. KJH5- 851 and KJH9-371) and Ti and V are unfractionated relative to one another, they are probably not caused by condensation of Ti and V before SiC formation but rather indicate higher Si. This feature even dominates the average abundance pattern of all measured KJH grains, which, relative to that of KJB, has higher Si (Fig. 1a). Large Si enrichments are expected from a model of high-temperature He-burning in more massive AGB stars (Brown and Clayton, 1992) proposed to explain the Si-isotopic compositions of single SiC grains such as KJH. References: Amari S., Zinner E., and Lewis R. S. (1991a) Lunar Planet. Sci. (abstract) 22, 19. Amari S., Zinner E., and Lewis R. S. (1991b) Meteoritics (abstract) 26, 314. Amari S., Lewis R. S., and Anders E. (1992) Geochim. Cosmochim. Acta, submitted. Brown L. E. and Clayton D. D. (1992) Ap. J. Lett., in press. Fegley B. Jr. (1991) Meteoritics (abstract) 26, 333. Gallino R., Busso M., Picchio G., and Raiteri C. M. (1990) Nature 348, 298-302. Kappler F., Gallino R., Busso C. M., Picchio G., and Raiteri C. M. (1990) Ap. J. 354, 630-643. Lattimer J. M., Schramm D. N., and Grossmann L. (1978) Ap. J. 219, 230-249. Lewis R. S., Amari S., and Anders E. (1990) Nature 348, 293-298. Lewis R. S., Amari S., and Anders E. (1992) Geochim. Cosmochim. Acta, submitted. Zinner E., Amari S., and Lewis R. S. (1991) Ap. J. 382, L47-L50.
Amari Sachiko
Hoppe Peter
Lewis Reed S.
Zinner Emst
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