Gmhimica e-t Cosmochimica Acla Vol. 58, pp. 47 1-494 Copyri& 0 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ekvier Science Ltd. F’rinted in U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0016-7037/93/56.@‘3 + .lXl Interstellar grains in meteorites: II. Sic and its noble gases ROY S. LEWIS, SACHIKO AMARI, * and EDWARDANDERS+ Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, IL 60637-1433, USA (Received July 11, 199 1; accepted in revised form July 29, I993 ) Abstract-We have analyzed He, Ne, Ar, Kr, and Xe in fourteen size fractions of interstellar Sic, isolated from the Murchison C2 chondrite. All are mixtures of a highly anomalous component bearing the isotopic signature of the astrophysical s-process and a more normal component, generally solar-like but with anomalies of up to 30% in the heavy isotopes. As these two components strikingly resemble predictions for the He-burning shells and envelopes of red giant carbon stars, it appears that the Sic grains are pristine circumstellar condensates from such stars. A number of elemental and isotopic ratios (such as Kr8’/Krs2 and Kr86/Kr82) vary with grain size, suggesting that the Sic comes from carbon stars representing a range of masses, metallicities, temperatures, and neutron densities. The Ne*‘-content of the Sic suggests a presolar cosmic-ray irradiation of up to 130 Ma, representing the interval between formation of the grains in a circumstellar shell and arrival in the solar system 4.6 Ga ago. Actually there is evidence that most of the Ne” (and NeZ2) is in I 10% of the grains, suggesting that much of the Sic was degassed during or shortly before formation of the solar system. Thus the true cosmic-ray ages may be 7 to 18X longer. Apparently the gas-rich SIC grains predate the solar system by at least 130 Ma and possibly up to 2000 Ma. 1. INTRODUCTION Meteoritic SIC, first isolated by TANG and ANDERS ( 1988a), has many isotopic anomalies, with up to 103-fold differences from terrestrial ratios. Several show the signature of the astrophysical s-process (neutron capture on a slow time scale), and thus point to an origin in red giant carbon stars, or “AGB stars” (asymptotic giant branch; IBEN and RENZINI, 1983; GALLINOet al., 1990). The s-process anom- alies include Xe-S ( SRINIVASAN and ANDERS, 1978), Kr-S ( ALAERTS et al., 1980; OTT et al., 1988), Bu (OTT and BEGE- MANN, 1990a; PROMBO et al., 1991, 1993; ZINNER et al., 1991a), Sr (OTT and BEGEMANN1990b), C’u (ZINNER et al., 1991b), Ti (IRELAND et al., 1991), Nd (ZINNER et al., 199la),and&z(AMARIetal., 1991a).Neon_E(H)ishighly enriched in Ne22 (LEWIS et al., 1979; EBERHARDT et al., 1979), possibly from the decay of 2.6 a NaZ2 (CLAYTON, 1975), and Mg shows a similar enrichment in Mg26, which isattributedto7 X 105aA126(Z~~~~~etal., 1990b, 1991~). The major elements of Sic also show large isotopic variations, from 2.7X for Si to - 1000X for C and N (ZINNER et al., 1987, 1989, 1991b; AMARI et al., 1991b; HOPPE et al., 1993; see ANDERSand ZINNER, 1993, for a review). It appears that Sic contains several distinct grain popu- lations. First, silicon and carbon isotopic compositions of at least the coarsest grains fall into several discrete clusters (ZINNER et al., 1989, 1991b; VIRAG et al., 1992). Second, the Xe-S/Ne-E ratio varies with grain size (TANG and AN- DERS, 1988a; ZINNERet al., 1989), and third, the proportions of KrsO.86 vary with release temperature of the gas on com- bustion (OTT et al., 1988) or heating (LEWIS and AMARI, * Present address: McDonnell Center for the Space Sciences, Washineton Universitv. St. Louis MO 63 130-4899.USA. t Present address: Gint. Engehaldenstrasse 12j20, 3004 Bern, Switzerland. 1989). The variations of Kr*’ and Kr86 reflect branching of the s-process at their radioactive progenitors, Se79 and Kr*’ (OTT et al., 1988). These branchings depend sensitively on neutron density and temperatures in the s-process region (OTT et al., 1988; BEER and MACKLIN, 1989; GALLINO et al., 1990), and thus can provide clues to the stars where the SIC formed. The anomalous noble gases in Sic separates are diluted by other, more normal, noble-gas components, which are located at least in part in other phases such as spine1 or dia- mond. We have therefore prepared two highly purified Sic samples, KJ and LQ, using a new procedure (AMARI et al., 1993 ) . The SiC samples were separated into nine or five grain- size fractions, which were analyzed for all five noble gases (KJ) or Ne, Kr, and Xe only (LQ). These data are consid- erably improved over previous analyses and lead to some major new insights. Some of the results, as well as results for other fractions from the new procedure, have been previously reported in a brief paper (LEWIS et al., 1990a) or abstracts ( AMARI and LEWIS, 1989; LEWIS and AMARI, 1989; VIRAG et al., 1989; WOPENKA et al., 1989; AMARI et al., 1990a,b; LEWIS et al., 1990b; ZINNERet al., 1990a). The present paper covers only noble-gas studies on Sic; noble gas data on dia- mond and graphite as well as ion-probe data on all phases are published separately. 2. MATERIALS AND METHODS 2.1. Samples Preparation and properties of the samples have been de- scribed by AMARIet al. ( 1993). The parent Sic samples LQA (gross 2.4 ppm, net 1.8 ppm) and KJ (6.6 ppm) were sepa- rated by sedimentation during centrifugation into five or nine grain-size fractions, LQB-LQF and KJA-KJI. Both sets were >90% pure Sic according to SEM-EDX analysis, but the L series samples were smaller and had lower noble-gas concen- 471