A transmission electron microscopy study of presolar spinel

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 (~1.2-1.4M_·) 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 sub-grains (<100nm) whose lattice structures are coherent with their host crystals. Oxygen isotope measurements of the Orgueil residue identified four additional particles all with similar elemental and isotope composition. These are the first known presolar Fe-Cr-rich spinels. The isotopic and microstructural data indicate that the Al-Mg-rich and Fe-Cr-rich grains experienced different condensation and processing histories. The single-crystal, stoichiometric, nearly pure Mg-Al spinels are generally consistent with equilibrium condensation predictions, which constrain their condensation temperatures between 1161 and 1221K, assuming total gas pressures of 1×10^-6 and 1×10^-3atm, respectively. Minor stacking disorder is observed in one of the Mg-Al spinels and is probably a result of slight perturbations to crystal growth during condensation in the circumstellar environment or of impact-induced sheer strain as a response to grain-grain collisions, which could have occurred in the circumstellar environment, the interstellar medium, or the solar nebula. The minor Ca in one of the Mg-Al spinels suggests back reaction with the circumstellar gas from which it formed. In comparison, the similarly oriented Fe-Cr-rich grains of the Orgueil assemblage and their homogeneous isotopic compositions are consistent with their condensation as a single circumstellar dust particle. However, the Fe-Cr-rich compositions (nearly chromite) are inconsistent with predictions for equilibrium condensation and suggest a complex cooling history that is not possible to precisely constrain.