Micrometeorites and extraterrestrial He in a ferromanganese crust from the Pacific Ocean

Extraterrestrial helium (Heet) in ferromanganese crusts from the deep oceans is presumed to originate in micrometeorites. The carrier phase of the Heet has, so far, not been identified. To address this issue we have recovered and examined several metallic micrometeorites from crust 237KD from the deep Pacific Ocean, and have performed He isotope determinations on powdered samples of the crust. A peak of release of extraterrestrial helium (Heet) at 600–800 °C is observed that is similar to the release peak of micrometeorite-bearing deep ocean sediments. The Heet content of acid-leached ferromanganese crust is approximately 70% less than that in unleached crust, implying that the bulk of the extraterrestrial 3He is hosted by a metallic phase rather than the refractory phases that host primordial He in meteorites. These observations support a micrometeoritic source for the Heet in 237KD. The time-integrated flux of extraterrestrial He measured in the ferromanganese crust is a small fraction of that determined from Pacific Ocean sediments, implying that the crust has trapped only a small fraction of the extraterrestrial dust that has fallen to Earth. The presence of metallic micrometeorites provides qualitative support to the contention that the 60Fe previously measured in crust 237KD may be meteoritic in origin.