SILICON ISOTOPIC COMPOSITION IN LARGE METEORITIC SIC PARTICLES AND NA-22 ORIGIN OF NE-22

Large silicon carbide (SiC) particles extracted from acid-insoluble residues of carbonaceous chondrites are isotopically anomalous in both silicon and carbon and contain isotopically extreme noble gases. These particles are thought to have originated in mass outflows from red giant stars and to have existed in the interstellar medium at the time the solar system formed from an interstellar cloud. Calculations show that the silicon isotope correlations in those large SiC particles can be generated only in the most massive carbon stars. Consequently, the almost pure neon-22 (Ne-22) in those particles must be interpreted as the condensation of radioactive sodium-22 (Na-22) in the particles as they flowed away from the stars. The Na-22 is produced through proton capture by Ne-21 at the base of the surface convection zone. Neon-22 does not exist abundantly in helium shells hot enough to burn magnesium, which is necessary to establish the measured silicon isotopic composition.