Single grain noble gas analysis of Antarctic micrometeorites by stepwise heating method with a newly constructed miniature furnace

摘要

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/cm ~3 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) cm ~3 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 ~(36)Ar/ ~(132)Xe 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.