河北蓟县常州沟组宇宙尘落地年龄

THE FALLING AGE OF COSMIC DUST FROM THE CHANGZHOUGOU FORMATION IN THE JIXIAN AREA,HEBEI

河北蓟县常州沟组铁质宇宙尘,形状多样,具气印、球粒、壳层和骸晶等多种结构。含钴、富硅、低镍。由磁铁矿、方铁矿、铁纹石、铁橄榄石等矿物组成,属陨石消融型。^(40)As/^(39)Ar阶段加热年龄谱呈单调上升型式,给出1670Ma形成年龄和170Ma热扰动时代。

Cosmic dust has been found in the strong magnetic fraction of heavy mineralsfrom the Changzhougou Formation in the Jixian standard section. It is mostabundant at 50 m below the formation top. It occurs mainly as single spherules,ellipsoids and spherule, chains and in the irregular shape. The surface of the cosmicdust is uneven, with pits and elongated grooves. On the section the hollow andsingle-nucleus double-layered structure often occur, which can be distinctly dividedinto three parts, crust, middle mantle and nucleus. The grain size of the cosmic spherules ranges generally from 0.1 to 0.3 mm,and reaches a maximum of 0.55 mm and a minimum of 0.02 mm. The fusion crustis composed of magnetite, wustite or rayalite and the nucleus consists of kamacite.The chemical composition is inhomogeneous, being Fe--rich, high--Si, low-Ti andlow-Ni. The fusion crust consists chiefly of Fe with subordinate Al, Si and minorTi, Ca and Mg. Its REE pattern shows that it has normal Eu, with δ Eu= 0.48-0.98. It is considered to be of meteorite fusion type. ^(40)Ar/^(39)Ar geochronological studies show that its age spectrum is of monotonous-rising type, giving the last thermal disturbing age of 170 Ma and a formation ageof 1670 Ma.