Rb-Sr features of the impact-melted LL-chondrites from Antarctica:Yamato-790723 and Yamato-790528

Rubidium and strontium abundances as well as Sr isotopic compositions were analyzed for the impact-melted meteorites (LL-chondrite : Yamato-790723 and -790528) from Antarctica to clarify their Rb-Sr features and to date the impact events. The samples were separated according to the magnetic property. The less magnetic and glass-rich portions of Y-790723 approximate an isochron of 1.27±0.19 Ga. The obtained age should indicate the impact event. The magnetic portions and fine-grained one largely deviate from the isochron. Y-790528 is rather depleted in K_2O and Rb, but not in Na_2O; this may be due to scarcity of the K-rich fragments. The ^Sr/^Sr ratios of the separated portions from Y-790528 are low and less variable. The five less magnetic portions of the Y-790528 chondrite seem to form the isochron of 4.36±0.76Ga as the time of the impact event. The large errors may have derived from local reset of the Rb-Sr chronometer and incomplete separation into the portions

Rb-Sr isotopic study of Yamato-794046 chondrite and its inclusion

The Yamato-794046 H chondrite and its inclusion were dated by the Rb-Sr method as part of a consortium study of unique inclusions in ordinary chondrites. Although the inclusion recorded an approximate 3.9 Ga-melting event, the age of the impact event of the host chondrite was unclear. The inclusion is not depleted in volatile elements such as Rb and the alkalis, even though the host is depleted. Therefore, when they were impacted, they should have been in different places. The host might have been impacted under low confining pressures, and the inclusion, in contrast, was melted under relatively high confining pressures