Circumstellar Fe oxide from the Acfer 094 carbonaceous chondrite

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 wtistite (FeO), but may contain a small amount of Mg in addition to Fe and O. The solid solution series magnesiowtistite, (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 magnesiowtistite 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.