Extreme Mg-26 and O-17 enrichments in an Orgueil corundum: Identification of a presolar oxide grain

A corundum (Al2O3) grain from the Orgueil meteorite is greatly enriched in O-17 and (Mg-26)*. The measured O-16/O-17 is 1028 +/- 11 compared to solar(O-16/O-17) = 2610. This is the largest O-17 excess so far observed in any meteoritic material. The Mg-26 excess ((Mg-26)*) is most plausibly due to in situ decay of Al-26. The inferred (Al-26/Al-27)0 ratio of 8.9 x 10-4 is approximately 18 times larger than the 5 x 10-5 value commonly observed in refractory inclusions formed in the solar system. The large O-17 excess and high (Mg-26)* Al-27 ratio unambiguously identify this corundum as a presolar oxide grain. Enrichments in O-17 and Al-26 are characteristic of H-burning and point to red giant or asymptotic giant branch (AGB) stars as likely sources.