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 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 ¹²C/¹³C ratios from 11 to 120, ¹⁴N/¹⁵N ratios between 1100 and 19000, initial ²⁶Al/²⁷Al ratios of less than 0.003, and, relative to solar system Si, deficits in ²⁹Si of up to 150 %‰ and enrichments in ³⁰Si of up to 510 %‰. 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 M_⊙), low-metallicity AGB stars that experienced strong cool bottom processing during the red giant phase.