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

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.