A transmission electron microscopy study of presolar spinel

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ScienceDirect Geochimica et Cosmochimica Acta 124 (2014) 152–169 www.elsevier.com/locate/gca

A transmission electron microscopy study of presolar spinel Thomas J. Zega a,b,e,⇑, Larry R. Nittler c, Frank Gyngard c,d, Conel M.O’D. Alexander c, Rhonda M. Stroud e, Ernst K. Zinner d a

Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, AZ 85721-0092, United States b Department of Materials Science and Engineering, University of Arizona, Tucson, AZ, United States c Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, DC 20015, United States d Laboratory for Space Sciences and Physics Department, Washington University, One Brookings Drive, Physics Department, Campus Box 1105, St. Louis, MO 63130, United States e Materials Science and Technology Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, United States Received 6 November 2012; accepted in revised form 9 September 2013; available online 25 September 2013

Abstract We report on the isotopic and microstructural properties of four presolar spinel grains identified in acid-resistant residues of the Murray CM2 and Orgueil (ORG) CI1 chondrites, and a mixture of the unequilibrated ordinary chondrites (UOC) QUE 97008 (L3.05), WSG 95300 (H3.3), and MET00452 (LL3.05) collected in Antarctica. All four grains have O-isotopic compositions indicating an origin in low-mass (1.2–1.4 M ) O-rich asymptotic giant branch (AGB) stars, although two of the grains have compositions indicating that non-standard mixing (cool-bottom processing) likely occurred in their parent stars. Three of the grains are single-crystal Mg–Al-rich spinels containing minor Fe and Cr; one is Mg deficient and one contains minor Ca. The fourth consists of an assemblage of three, Fe–Cr-rich crystalline grains with closely aligned crystallographic orientation but systematically varied cation composition. Each spinel grain within the assemblage also contains Ti-rich subgrains (