Isotopic Compositions and Elemental Abundances of Chromium in Presolar Silicon Carbide Grains from Asymptotic Giant Branch Stars

Abstract Isotopes of chromium (Cr) and other iron-group elements are predominantly made in the inner shells of supernovae and are later reprocessed via the slow neutron capture process ( s -process) in asymptotic giant branch (AGB) stars. Nucleosynthetic models of low-mass, solar metallicity AGB stars yield significant overproduction of 54 Cr ( δ 54 Cr values ∼ +160‰) with limited variations in other Cr isotopes, relative to solar system values. Here we report Cr, C, and N isotopic compositions of 16 individual presolar silicon carbide (SiC) grains of the KJG series (1.5–3 μ m) from the Murchison (CM2.0) meteorite. 12 C/ 13 C and 14 N/ 15 N ratios of the 14 mainstream SiC grains range from 29 to 108 and 259 to 7800, respectively. These C, N isotopic compositions are consistent with their formation in red giant and AGB stars. Two types of AB grains could have originated in a J-type C-star (AB2) and a type-II supernova (AB). The majority of mainstream grains display close-to-solar Cr isotopic compositions, indicating the Cr was not significantly processed within their parent AGB stars. A mainstream SiC grain with relatively high 54 Cr enrichment of δ 54 Cr ∼ 700‰ likely originated from a very low metallicity parent star based on the stellar nucleosynthesis model (FRUITY). We consider the plausible origin of this grain from Galactic halo stars, migration from the outer Galactic disk, and other scenarios. Elemental Cr concentrations of the grains vary from ∼1 to 9 ppm, with 75% of the grains displaying an average concentration of ∼2 ppm. Cr concentration does not vary significantly with grain size, suggesting that the Cr in the SiC grains condensed during grain formation and was not implanted at a later stage.