New insights into the carrier phase(s) of extraterrestrial He in geologically old sediments

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial ³He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ∼90 Myr. The leaching experiments suggest that the host phase(s) of ³He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al₂O₃, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the ³He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the ³He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth’s surface for the same amount of time would lose up to 60%. The absolute magnitude of the ³He loss is, however, likely to depend upon the ³He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 μm in diameter are responsible for the extraterrestrial ³He in geologically old sediments Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial ³He flux to earth. Geochim. Cosmochim. Acta63, 2653-2665], our sieving experiment demonstrates that at most 20% of the ³He is carried by particles greater than 50 μm in diameter. The size-distribution of the ³He-bearing particles implies that extraterrestrial ³He in sediments record the IDP flux rather than the micrometeorite flux.