Isotopic Composition of Silicon Carbide in the Co3 Chondrite Colony

Introduction: Presolar grains have been identified in primitive members of all chondrite classes [1,2]. The isotopic compositions of presolar grains provide probes of galactic evolution and nucleosynthesis in stars [3,4], while the abundances and characteristics of presolar grains contain a record of thermal processing in the solar system [2,5]. Most of the detailed isotopic work has been done on SiC from Murchison [e.g., 2] and Orgueil [4], supplemented by a few studies of ordinary and enstatite chondrites [6-9]. This work investigates SiC in the Colony CO3.O meteorite. SiC is present in Colony at a matrix-normalized abundance of ~3.7 ppm [5], much less than in CI chondrites and the matrixes of CM and primitive ordinary and enstatite chondrites [3]. The abundances of SiC and other presolar grains in Colony seem to correlate with the chemical processing that produced CO3 chondrites [5,10]. This implies that the known presolar grains experienced the same processing as the bulk CO3 material and assumes that the parent material was the same as that for other chondrites, including CI. That parent material is most plausibly the average material in the sun’s parent molecular cloud. One test of this idea is to look for primary differences between SiC in Colony (and other meteorites) and that in CI and CM chondrites. True differences not related to thermal processing would falsify the assumption that all chondrite classes originated from the same reservoir of presolar dust [5]. Techniques: A Colony sample of ~2.5 grams from the ASU collection was processed using the methodology developed by [2]. The SiC-spinel residue was dispersed onto gold foils and SiC grains were identified using a JEOL 845 SEM equipped with an energydispersive X-ray system and an automated grainidentification program provided by IXRF systems. Using techniques described previously [4], 22 SiC grains were measured for Si and C isotopes, and 12 of these were measured for N isotopes with the ASU Cameca ims 6f ion probe. Isotopic Results: Silicon isotopic data (Fig. 1) show a distribution very similar to that exhibited by CI, CM, ordinary, and enstatite chondrites [3,4,6-7]. Most of the data scatter along the “mainstream” array of slope ~2.2 (~1.4 in delta coordinates) that does not pass through the solar composition. One grain at the far right of Fig. 1 appears to have a large excess of sprocess Si. Another grain, in the bottom center of Fig. 1, apparently comes from a low-metallicity star and exhibits a moderate contribution of s-process Si. No grains with large Si excesses (X-grains) were observed among the measured grains.