STELLAR SAPPHIRES - THE PROPERTIES AND ORIGINS OF PRESOLAR AL2O3 IN METEORITES

Thirty-seven isotopically highly anomalous presolar Al2O3 grains and o ne presolar MgAl2O4 grain from a separate of the Tieschitz H3.6 ordina ry chondrite were identified out of 17,000 isotopically normal refract ory oxide grains by an automatic O-16/O-18 low mass resolution ion-ima ging mapping technique in the ion microprobe. Eight additional presola r Al2O3 grains were found by high mass resolution ion probe measuremen ts of all three stable O isotopes in individual grains, including seve ral that would have been missed by the ion-imaging search. Forty-five of the grains were analyzed for their O-16/O-17 and O-16/O-18 ratios. Twenty-four grains were also analyzed for Al-Mg and 17 of them have la rge excesses of Mg-26, attributable to the radioactive decay of Al-26. The highly anomalous isotopic composition of the grains is evidence f or their presolar, stellar origin. The 46 oxide grains of this study t ogether with 42 previously identified presolar grains were divided int o four groups. These groups most likely comprise grains from distinct types of stellar sources. Group 1 grains have O-17 excesses and modera te O-18 depletions, relative to solar, and many of them exhibit Mg-26 excesses as well. Group 2 grains have O-17 excesses, large O-18 deplet ions, and high inferred Al-26/Al-27 ratios. Group 3 grains have solar or higher O-16/O-17 and O-16/O-18 ratios. Group 4 grains have O-17 and O-18 enrichments. One Al2O3 grain of this study, T54, has an O-16/O-1 7 ratio of 71, lower than any previously observed, and O-16/O-18 much greater than the solar value. The O-isotopic compositions of Group 1 a nd Group 3 grains are consistent with an origin in O-rich red giant st ars, which have undergone the first dredge-up. The range of O-isotopic ratios of these groups requires multiple stellar. sources of differen t masses and initial isotopic compositions and is well explained by a combination of Galactic chemical evolution and first dredge-up models. The inferred Al-26/Al-27 ratios of many of these grains indicate that they formed in thermally pulsing asymptotic branch (TP-AGB) stars tha t had undergone the third dredge-up. Group 2 grains probably formed in low-mass AGE stars as well, and their substantial O-18 depletions are the likely result of ''extra'' mixing (cool bottom processing). The o rigin of the O-18 enrichments in Group 4 grains is unknown, but it mig ht be due to initial compositional differences of the stellar sources or to unusual third dredge-up in low-mass AGE stars. The highly O-17-e nriched grain T54 could have formed in an AGE star undergoing hot bott om burning or in a massive star in the Of-WN phase. O-rich circumstell ar dust seems to be underrepresented in meteorites, relative to C-rich . Explanations include the possibility that most O-rich stardust grain s 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.