Inferred Initial 26Al/27Al Ratios in Presolar Stardust Grains from Supernovae are Higher than Previously Estimated

We performed an in-depth exploration of the Al–Mg system for presolar graphite, SiC, and Si3N4 grains found to contain large excesses of 26Mg, indicative of the initial presence of live 26Al. Ninety of the more than 450 presolar grains processed in this study contain well-correlated ${\delta }^{26}\mathrm{Mg}{/}^{24}\mathrm{Mg}$ and 27Al/24Mg ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial ${}^{26}\mathrm{Al}{/}^{27}\mathrm{Al}$ 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 ${\delta }^{26}\mathrm{Mg}{/}^{24}\mathrm{Mg}$ 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 ${}^{26}\mathrm{Al}{/}^{27}\mathrm{Al}$ ratios. The presolar grains with the highest ${}^{26}\mathrm{Al}{/}^{27}\mathrm{Al}$ ratios are from Type II supernovae whose isochron-derived initial ${}^{26}\mathrm{Al}{/}^{27}\mathrm{Al}$ ratios greatly exceed those predicted in the He/C and He/N zones of SN models.