A preliminary study of Rb-Sr systematics and trace element abundances on impact-melted LL-chondrites from Antarctica

Abstract

Rb-Sr systematics, REE, Ba, Sr, Rb, K, Ca and Mg abundances were analyzed in 1) whole-rock samples from 8 Antarctic LL-chondrites including 4 impact-melted rocks and 2 non-Antarctic LL chondrites, and 2) 8 mineral separates from one of the impact-melted meteorites, Y-790964. In a ^(Rb)-^(Sr) evolution diagram, analyses from the severely shocked meteorites deviate from the 4.5 b. y.-evolution line although those of normal LL-chondrites are plotted on or close to the line, suggesting a late thermal evolution of these impact-melted meteorites. Analyses of mineral separates and a whole-rock from Y-790964 yield a Rb-Sr internal isochron age of 1197±54 (2σ) m. y. and an initial ^(Sr)/^(Sr) ratio of 0.73160±0.00028 (2σ). This age is the youngest known among impact-related materials in brecciated meteorites. In addition, the impact-melted meteorites have somewhat higher and fractionated REE, Sr, Rb and K abundances compared with normal LL chondrites. The 1.2 b. y. age is interpreted as a time of melting induced by intense impacts on the LL-chondrite parent body, accompanied by REE, Rb/Sr and K fractionations. Therefore, we suggest that strong impact and regolith processes on the LL-chondrite parent body never ceased until at least 1.2 b. y. ago