Iron Implantation in Presolar Supernova Grains

We consider the potential of measured iron isotopic ratios within presolar grains from supernovae (as discovered in meteorites) for identifying the gas from which the grains condensed. We show that although iron isotopic ratios vary dramatically with radial coordinate in the initial supernova, it seems likely that the concentration of iron that thermally condenses in SiC grains within the supernova interior may be smaller than the concentration that will later be implanted by high-speed grain-gas collisions following the penetration of the reverse shock into the supernova flow. In that case, the Fe isotopic composition is much altered. We propose that the ⁵⁸Fe richness that is very evident in the three SiC grains analyzed to date is the result of ion implantation during the grain's rapid radial motion through the shocked and decelerated overlying supernova gas that is ⁵⁸Fe-rich. We point to other likely applications of this same idea and speculate that only the dominant isotopes of the SiC grains, namely ²⁸Si and ¹²C, can be safely assumed to be initial thermal condensate. We conclude that a violent period of implantation plus sputtering has overprinted the initial thermal condensate. If correct, this points to a new technique for sampling the velocity mixing within young supernova remnants.