Search for the origin of exotic helium in deep-sea sediments

Merrihue¹ first observed helium with a very high ³He/⁴He isotope ratio ( ≥10^-4; here termed exotic helium) in a magnetic separate of a deep-ocean sediment and attributed it to input of extraterrestrial material. Krylov et al.² subsequently reported the existence of exotic He in several deep-sea sediments, while Tilles³ measured argon isotope ratios in magnetic separates of two ocean sediments. Observing that the ³He/⁴He ratio is inversely correlated with the sedimentation rate, Ozima et al.⁴ concluded that the exotic He is carried by interplanetary dust particles (IDPs). We have now examined other deep-sea sediments to characterize the exotic He and to identify its carrier, and have confirmed that the exotic He resides mainly in a strongly magnetic fraction. Stepwise degassing experiments suggest that the exotic He consists of a single component. The carrier of the He is likely to be chondritic IDPs. The higher level of exotic He in magnetic fractions of sediments can be explained by secondarily produced magnetite in IDPs as a result of thermal shock during atmospheric entry.