2015ApJ...809...31G

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 δ²⁶Mg/²⁴Mg and ²⁷Al/²⁴Mg ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial ²⁶Al/²⁷Al 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 δ²⁶Mg/²⁴Mg 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 ²⁶Al/²⁷Al ratios. The presolar grains with the highest ²⁶Al/²⁷Al ratios are from Type II supernovae whose isochron-derived initial ²⁶Al/²⁷Al ratios greatly exceed those predicted in the He/C and He/N zones of SN models.