INFERRED INITIAL26Al/27Al RATIOS IN PRESOLAR STARDUST GRAINS FROM SUPERNOVAE ARE HIGHER THAN PREVIOUSLY ESTIMATED

Abstract

We performed an in-depth exploration of the Al–Mg system for presolar graphite, SiC, and Si3N4grains found to contain large excesses of 26 Mg, indicative of the initial presence of live 26 Al. Ninety of the more than 450 presolar grains processed in this study contain well-correlated Mg Mg 26 24 d and 27 Al/ 24 Mg ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial Al Al 26 27 ratios that, on average, are ∼1.5–2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative Mg Mg 26 24 d intercepts of the isochron lines. Al contamination is potentially due to etching of the grains’ surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial Al Al 26 27 ratios. The presolar grains with the highest Al Al 26 27 ratios are from Type II supernovae whose isochronderived initial Al Al 26 27 ratios greatly exceed those predicted in the He/C and He/N zones of SN models.