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

摘要

Nine silicon carbide grains of the rare type Z separated from the Murchison CM2 meteorite have been analyzed for the isotopic compositions of C, Si, N (seven grains), and Mg-Al (two grains) by ion microprobe mass spectrometry. These grains have ~(12)C/~(13)C ratios from 11 to 120, ~(14)N/~(15)N ratios between 1100 and 19000, initial ~(26)Al/~(27)Al ratios of less than 0.003, and, relative to solar system Si, deficits in ~(29)Si of up to 150per thousand and enrichments in ~(30)Si of up to 510 per thousand. These isotopic signatures rule out the previously postulated nova or Type Ⅱ supernova origin of the Z grains. Based on the predictions from a new asymptotic giant branch (AGB) star model it appears likely that the Z grains formed in the outflows of low-mass ( < 2.3 solar mass), low-metallicity AGB stars that experienced strong cool bottom processing during the red giant phase.