New Insights into the Carrier Phase(s) of Extraterrestrial He-3 in Geologically Old Sediments

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Mukhopadhyay, Sujoy, Farley, Ken
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2006
• Subjects: Antarctic; Antarc*
• Online Access: http://nrs.harvard.edu/urn-3:HUL.InstRepos:3224736; https://doi.org/10.1016/j.gca.2006.06.1566

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 3 He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to similar to 90 Myr. The leaching experiments suggest that the host phase(s) of 3 He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al 2 O 3 , and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the 3 He 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 3 He 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 3 He loss is, however, likely to depend upon the 3 He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 mu m in diameter are responsible for the extraterrestrial 3 He 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 3 He flux to earth. Geochim. Cosmochim. Acta 63,2653-2665 ], our sieving experiment demonstrates that at most 20% of the 3 He is carried by particles greater than 50 mu m in diameter. The size-distribution of the 3 He-bearing particles implies that extraterrestrial 3 He in sediments record the IDP flux rather than the micrometeorite flux. Earth and Planetary Sciences Version of Record