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 O-17/O-16 ratio and an O-18/O-16 ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100 parts per thousand-200 parts per thousand in both Fe-54 and Fe-57, relative to Fe-56. No evidence of excess Ni-60 from the decay of extinct Fe-60 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 mu m 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.