Thermal history of the shock-melted Antarctic LL-chondrites from the Yamato-79 collection
Thermal history of the shock-melted Antarctic LL-chondrites from the Yamato-79 collection
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Yamato-79 收藏中受冲击融化的南极 LL 球粒陨石的热历史
DOI：
10.1016/0016-7037(90)90301-z
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发表时间：
1990
影响因子：
5
通讯作者：
K. Nagao
中科院分区：
文献类型：
--
作者：
O. Okano;N. Nakamura;K. Nagao
测定了一系列Yamato-79LL球粒陨石经冲击熔融后的锶、稀有气体同位素组成和亲石微量元素(K、Rb、Sr、Ba、Rees)的丰度，以研究其晚期热历史和冲击过程的化学特征。所有陨石具有相似的冲击年龄(~1.2Ga)，这是由Rb-Sr内等时线和K-Ar年龄、稀有气体成分和宇宙线暴露年龄(~28 Ma)、岩石学结构和南极采样点证实的。这些结果表明，所有这些陨石都是同一坠落的一部分。1.2Ga冲击事件导致强烈的(部分到全部)熔融，随后橄榄石和单斜辉石重结晶，泡化，冲击诱导的碱均一化，以及Rb-Sr体系的局部同位素平衡或扰动。冲击效应的程度因样本而异，因部分而异，甚至在单个样本中也是如此。铁在橄榄石中扩散的模型计算表明，冷热物质在母体的撞击喷射层中密切接触。根据这些模型计算和宇宙成因稀有气体成分提供的证据，得出结论：1.2Ga事件在撞击坑深处(>；2m，屏蔽宇宙射线)形成了由冷和热角砾物质组成的撞击熔体喷出物堆。在~28 Ma的附加碰撞中，母体被碎裂成米大小的石头，形成了Yamato-79冲击球粒陨石的母质。
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.