¹Thomas J. Zega, ^2,3Pierre Haenecour, ²Christine Floss, ⁴Rhonda M. Stroud
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, AZ 85721-0092, USA
²Laboratory for Space Sciences and Physics Department, Washington University, One Brookings Drive, Campus Box 1105, St. Louis, MO 63130, USA
³Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, Campus Box 1169, St. Louis, MO 63130, USA
⁴Materials Science and Technology Division, Code 6366, Naval Research Laboratory, 4555 Overlook Ave, SW Washington, DC 20375, USA

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.

Reference
Zega TJ, Haenecour P, Floss C, Stroud RM (2015) Circumstellar Magnetite from the LAP 031117 CO3.0 Chondrite. Astrophysical Journal 808 55.
Link to Article [doi:10.1088/0004-637X/808/1/55]