Circumstellar Fe Oxide from the Acfer 094 Carbonaceous Chondrite

Abstract

We report the discovery of a unique Fe- and O-bearing circumstellar grain from the Acfer 094 ungrouped carbonaceous chondrite. The grain has a close-to-solar 17O/16O ratio and an 18O/16O ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100‰-200‰ in both 54Fe and 57Fe, relative to 56Fe. No evidence of excess 60Ni from the decay of extinct 60Fe was observed. Auger elemental spectra show that the grain is compositionally similar to wustite (FeO), but may contain a small amount of Mg in addition to Fe and O. The solid solution series magnesiowustite, (Mg,Fe)O, is predicted to form under nonequilibrium conditions in oxygen-rich asymptotic giant branch (AGB) stars with low mass-loss rates, and Fe-rich magnesiowustite has been proposed as the carrier of the 19.5 μm feature observed in the spectra of certain low-mass-loss AGB stars that show little silicate emission. Although the isotopic data are ambiguous, these observations argue in favor of an AGB source for this Fe oxide grain.