Presolar corundum and spinel in ordinary chondrites: Origins from AGB stars and a supernova

On the basis of anomalous isotopic compositions of oxygen and magnesium, 14 oxide grains from two primitive meteorites (Bishunpur and Semarkona) have been identified as circumstellar condensates. One corundum grain has a high (18)O/(16)O ratio and isotopic compositions of magnesium, calcium, and titanium that are compatible with a formation in ejecta of a type II supernova that was about 15 times the mass of the sun, The other grains have oxygen, magnesium, and titanium compositions that are consistent with a formation around asymptotic giant branch (AGB) stars with a range of mass and initial composition. The large range of aluminum/magnesium in circumstellar corundum and spinel is considered to reflect various stages of back-reaction between condensed corundum and gaseous magnesium in cooling stellar ejecta.