Keisuke Nagao

Noble gas studies on the host phase of high 3He4He ratios in deep-sea sediments

Abstract Elemental abundances and isotopic compositions of noble gases were determined for a magnetic separate and chemically leached samples from deep-sea sediments (14°29.61′S, 158°52.98′W). High 3 He 4 He ratios suggesting the presence of cosmic material were observed in all samples, and especially enriched in magnetic fractions. Ne isotopes and light noble gas abundances suggested that the high 3 He 4 He ratio was of solar wind origin and not due to cosmic ray spallation. However, heavy noble gases (Kr and Xe) were of atmospheric origin. Altogether the noble gases were well explained by a mixture of solar wind implanted on interplanetary dust particles (IDPs) and terrestrial noble gas fractionated from the atmosphere. The degree of contamination of IDPs in the magnetic fractions was estimated to be about 100 ppm.

Thermal history of the shock-melted Antarctic LL-chondrites from the Yamato-79 collection

Abstract The Sr and rare gas isotopic compositions and abundances of lithophile trace elements (K, Rb, Sr, Ba, and REEs) were determined for a series of shock-melted Yamato-79 LL-chondrites to investigate their late thermal history and the chemical features of shock processes. All meteorites show similarities in shock ages (~ 1.2 Ga) as confirmed by Rb-Sr internal isochron and K-Ar dating, rare gas compositions as well as cosmic-ray exposure ages (~28 Ma), petrographie textures, and sampling sites in Antarctica. These results indicate that all of these meteorites are part of the same fall. The 1.2 Ga shock event caused a severe (partial to total) melting followed by recrystallization of olivine and clinopyroxene, vesiculation, shock-induced alkali homogenization, and local isotopic equilibration or perturbation of the Rb-Sr system. The degrees of shock effects are variable from specimen to specimen and from portion to portion, even in a single specimen. Model calculations of Fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body. From these model calculations and the evidence provided by cosmogenic rare gas compositions, it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth (> 2 m, shielded from cosmic rays) in an impact crater by the 1.2 Ga event. The parent body was fragmented to meter-size stones by an additional collision at ~28 Ma resulting in the formation of the parent material of the Yamato-79 shocked chondrites.