Galactic-Cosmic-Ray-ProducedHe3in a Ferromanganese Crust: Any SupernovaFe60Excess on Earth?

An excess of ${}^{60}\mathrm{Fe}$ in 2.4–$3.2\times {10}^6$ 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 ${}^3\mathrm{He}/{}^4\mathrm{He}$ (up to $6.12\times {10}^{-3}$) and ${}^3\mathrm{He}$ concentrations (up to $8\times {10}^9\mathrm{\text{atoms}}/\mathrm{g}$) 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. ${}^{60}\mathrm{Fe}$ is produced by GCR reactions on Ni in extraterrestrial material. The maximum ${}^3\mathrm{He}/{}^{60}\mathrm{Fe}$ of 237KD (80–850) is comparable to the GCR ${}^3\mathrm{He}/{}^{60}\mathrm{Fe}$ production ratio (400–500) predicted for Ni-bearing minerals in iron meteorites. The excess ${}^{60}\mathrm{Fe}$ can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source.