Stellar Sapphires: The Properties and Origins of Presolar Al₂O₃ in Meteorites

Thirty-seven isotopically highly anomalous presolar Al₂O₃ grains and one presolar MgAl₂O₄ grain from a separate of the Tieschitz H3.6 ordinary chondrite were identified out of 17,000 isotopically normal refractory oxide grains by an automatic ¹⁶O/¹⁸O low mass resolution ion-imaging mapping technique in the ion microprobe. Eight additional presolar Al₂O₃ grains were found by high mass resolution ion probe measurements of all three stable O isotopes in individual grains, including several that would have been missed by the ion-imaging search. Forty-five of the grains were analyzed for their ¹⁶O/¹⁷O and ¹⁶O/¹⁸O ratios. Twenty-four grains were also analyzed for Al-Mg and 17 of them have large excesses of ²⁶Mg, attributable to the radioactive decay of ²⁶Al. The highly anomalous isotopic composition of the grains is evidence for their presolar, stellar origin.

The 46 oxide grains of this study together with 42 previously identified presolar grains were divided into four groups. These groups most likely comprise grains from distinct types of stellar sources. Group 1 grains have ¹⁷O excesses and moderate ¹⁸O depletions, relative to solar, and many of them exhibit ²⁶Mg excesses as well. Group 2 grains have ¹⁷O excesses, large ¹⁸O depletions, and high inferred ²⁶Al/²⁷Al ratios. Group 3 grains have solar or higher ¹⁶O/¹⁷O and ¹⁶O/¹⁸O ratios. Group 4 grains have ¹⁷O and ¹⁸O enrichments. One Al₂O₃ grain of this study, T54, has an ¹⁶O/¹⁷O ratio of 71, lower than any previously observed, and ¹⁶O/¹⁸O much greater than the solar value.

The O-isotopic compositions of Group 1 and Group 3 grains are consistent with an origin in O-rich red giant stars, which have undergone the first dredge-up. The range of O-isotopic ratios of these groups requires multiple stellar sources of different masses and initial isotopic compositions and is well explained by a combination of Galactic chemical evolution and first dredge-up models. The inferred ²⁶Al/²⁷Al ratios of many of these grains indicate that they formed in thermally pulsing asymptotic branch (TP-AGB) stars that had undergone the third dredge-up. Group 2 grains probably formed in low-mass AGB stars as well, and their substantial ¹⁸O depletions are the likely result of "extra" mixing (cool bottom processing). The origin of the ¹⁸O enrichments in Group 4 grains is unknown, but it might be due to initial compositional differences of the stellar sources or to unusual third dredge-up in low-mass AGB stars. The highly ¹⁷O-enriched grain T54 could have formed in an AGB star undergoing hot bottom burning or in a massive star in the Of-WN phase.

O-rich circumstellar dust seems to be underrepresented in meteorites, relative to C-rich. Explanations include the possibility that most O-rich stardust grains are silicates and have been destroyed either in the laboratory or in nature and the possibility that presolar Al₂O₃ has a finer grain size distribution than SiC and graphite.