Galactic-Cosmic-Ray-Produced ~3He in a Ferromanganese Crust: Any Supernova ~(60)Fe Excess on Earth?

摘要

An excess of ~(60)Fe in 2.4-3.2 X 10~6 year old ferromanganese crust (237KD) from the deep Pacific Ocean has been considered as evidence for the delivery of debris from a nearby supernova explosion to Earth. Extremely high ~3He/~4He (up to 6.12 X 10~(-3)) and ~3He concentrations (up to 8 X 10~9 atoms/g) measured in 237KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR) and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. ~(60)Fe is produced by GCR reactions on Ni in extraterrestrial material. The maximum ~3He/~(60)Fe of 237KD (80-850) is comparable to the GCR ~3He/~(60)Fe production ratio (400-500) predicted for Ni-bearing minerals in iron meteorites. The excess ~(60)Fe can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source.