1,200-Myr impact-melting age and trace-element chemical features of the Yamato-790964 chondrite

Age determinations of impact-produced materials in brecciated meteorites¹ are particulary important for understanding the evolutionary history of meteorites. The age characteristics of these meteorites have been examined mostly by the K-Ar method or, in a few cases², by the Rb-Sr method. However, age data are still rare and, except for a few cases³, the large-ion lithophile (LIL) trace-element chemical features have not been studied in detail. In this respect, the Yamato-79 shocked meteorites are the best specimens for investigation. We report here results of Rb-Sr systematics and trace-element analyses for an almost wholly impact-melted LL-chondrite, Y-790964, one of the Yamato-79 unusual chondrites. The analyses yield an internal isochron age of 1,200±50 (2σ) Myr. The isotopic and trace-element data obtained represent the first clear evidence of the youngest impact-melting and LIL-element fractionations for a complete LL-chondrite, and suggest that the 1,200-Myr-age is a non-trivial marker in the evolution of meteorites.