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 17 O/ 16 O ratio and an 18 O/ 16 O ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100‰-200‰ in both 54 Fe and 57 Fe, relative to 56 Fe. No evidence of excess 60 Ni from the decay of extinct 60 Fe 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. © 2008. The American Astronomical Society. All rights reserved.