Earth Planets Space, Vol. 63 (No. 10), pp. 1097-1111, 2011
Ken-ichi Bajo1, Tomohiro Akaida1, Noriaki Ohashi2, Takaaki Noguchi2, Tomoki Nakamura3, Yoshinobu Nakamura1, Hirochika Sumino1, and Keisuke Nagao1
1Geochemical Research Center, Graduate School of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
2College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan
3Department of Earth Science, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
(Received September 27, 2010; Revised July 31, 2011; Accepted August 2, 2011; Online published February 2, 2012)
Ten micrometeorites weighing 0.14-18.5 μg, each retrieved from surface snow near the Dome Fuji Station, Antarctica (snow-AMMs), were studied to elucidate their noble gases, mineralogy, morphology, and chemical compositions. Low densities in the range of 0.2-1.4 g/cm3 estimated for seven samples suggested a porous inner structure. Noble gases were extracted from each particle using stepwise heating with a laboratory manufactured miniature furnace. Isotopic ratios of He and Ne indicate that the light noble gases with high 4He concentrations ranging from 10-2 to 10-4 cm3 STP/g are mostly of solar origin. The higher concentrations of 4He observed for several samples are comparable with those of IDPs enriched in solar He, but exceed those reported for ice-AMMs. In contrast to He and Ne, heavy noble gases Ar, Kr, and Xe are primordial ones resembling Q-gas trapped in chondrites, although a small contribution of solar Ar is indicated for some samples with higher 36Ar/132Xe ratios than that for the Q-gas. Three particles released appreciable amounts of He at temperatures lower than 800°C, suggesting heating temperatures lower than 700°C at the time of atmospheric entry. Other particles released at most 10% of total He at the temperatures up to 800°C. Based on their sizes, weights, and release profiles of 4He, initial speeds of less than 14 km/s at atmospheric entry were indicated for the particles. The slow entry speeds imply that all the snow-AMMs studied in this work were likely derived from asteroids. The present work demonstrates that the miniature furnace can be applicable to noble gas analysis of tiny grains from the Itokawa asteroidal regolith materials returned by the Hayabusa mission.
Key words: Noble gases, snow Antarctic micrometeorites, miniature furnace, stepwise heating, entry velocity.