METEORITIC SILICON-CARBIDE GRAINS WITH UNUSUAL SI-ISOTOPIC COMPOSITIONS - EVIDENCE FOR AN ORIGIN IN LOW-MASS, LOW-METALLICITY ASYMPTOTIC GIANT BRANCH STARS

Nine silicon carbide grains of the rare type Z separated from the Murc hison CM2 meteorite have been analyzed for the isotopic compositions o f C, Si, N (seven grains), and Mg-Al (two grains) by ion microprobe ma ss spectrometry. These grains have C-12/C-13 ratios from II to 120, N- 14/N-15 ratios between 1100 and 19000, initial Al-26/Al-27 ratios of l ess than 0.003, and, relative to solar system Si, deficits in Si-29 of up to 150 parts per thousand and enrichments in Si-30 Of UP to 510 pa rts per thousand. These isotopic signatures rule out the previously po stulated nova or Type II supernova origin of the Z grains. Based on th e predictions from a new asymptotic giant branch (AGE) star model it a ppears likely that the Z grains formed in the outflows of low-mass (<2 .3 M.), low-metallicity AGE stars that experienced strong cool bottom processing during the red giant phase.