TRACE-ELEMENT CONCENTRATIONS IN SINGLE CIRCUMSTELLAR SILICON-CARBIDE GRAINS FROM THE MURCHISON METEORITE

Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 i ndividual silicon carbide (SiC) grains (in addition, Nb and Nd were de termined in 20 of them), from separate KJH (size range 3.4-5.9 mu m) o f the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic c ompositions have been measured before (Hoppe ef al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC gr ains represent all previously recognized subgroups: mainstream (20 < C -12/C-13 < 120; 200 < N-14/N-15; Si isotopes on slope 1.34 line), grai ns A (C-12/C-13 < 3.5), grains B (3.5 < C-12/C-13 < 10), grains X (N-1 5 excesses, large Si-28 excesses) and grains Y (150 < C-12/C-13 < 260; Si isotopes on slope 0.35 line). Data on these grains are compared wi th measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion o f the condensation of trace elements in SiC from C-rich stellar atmosp heres is given in a companion paper by Lodders and Fegley (1995). Elem ents such as Mg, Al, Ca, Fe and Sr are depleted because their compound s are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically e nriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsi ng AGB stars) are the main source of circumstellar SiC grains. Large g rains are less enriched than small grains, possibly because they are f rom different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understoo d if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon sta rs that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how sta rs become carbon stars without much accompanying dredge-up of s-proces s elements.