Circumstellar Magnetite from the LAP 031117 CO3.0 Chondrite

Thomas J. Zega, Pierre Haenecour, Christine Floss, and Rhonda M. Stroud

The Astrophysical Journal Volume 808 Number 1
Thomas J. Zega et al. 2015 ApJ 808 55. doi:10.1088/0004-637X/808/1/55


We report the first microstructural confirmation of circumstellar magnetite, identified in a petrographic thin section of the LaPaz Icefield 031117 CO3.0 chondrite. The O-isotopic composition of the grain indicates an origin in a low-mass (~2.2 M?), approximately solar metallicity red/asymptotic giant branch (RGB/AGB) star undergoing first dredge-up. The magnetite is a single crystal measuring 750 × 670 nm, is free of defects, and is stoichiometric Fe3O4. We hypothesize that the magnetite formed via oxidation of previously condensed Fe dust within the circumstellar envelope of its progenitor star. Using an empirically derived rate constant for this reaction, we calculate that such oxidation could have occurred over timescales ranging from approximately ~9000?500,000 years. This timescale is within the lifetime of estimates for dust condensation within RGB/AGB stars.