ABSTRACT

Lunar meteorites recovered from Antarctica are recognized to have been derived from the lunar highlands. Seventeen samples (rock fragments and clasts) from Yamato-791197 and Yamato-82192 have been analyzed for chemical compositions (rare earth elements and major elements) and Rb-Sr systematics. Those samples show chemical and chronological characteristics resembling the lunar highland samples very much. The lunar highland rocks have been known to show positive Ce anomalies in their REE (rare earth elements) abundance patterns. These anomalies are considered to have formed on the lunar surface, but their genesis has not been discussed definitely. The lunar meteorites, which we analyzed, also show the positive Ce. anomalies. According to our geochemical (major elements and REE abundances) and chronological (Rb-Sr systematics) studies for some lunar meteorites, the Ce anomalies appear to increase with decreasing Al₂O₃ and CaO contents. Furthermore, the initial ⁸⁷Sr/⁸⁶Sr ratios obtained for some samples with ages of around 3.9 Gyr are as low as that for the Juvinas eucrite which has an age of 4.5 Gyr, indicating the removal of radiogenic ⁸⁷Sr. Samples of a lunar meteorite, which form an isochron with relatively high initial ratio, show smaller positive Ce anomalies than the samples of 2 other lunar meteorites which form isochrons with relatively low initial ratios. As one possibility, these correlations among major element contents, REE abundances, initial ⁸⁷Sr/⁸⁶Sr ratios and Ce anomalies suggest that H₂O (possibly ice) might have played a role in the alteration having produced the Ce anomalies, and it is suggested that ice may have existed at least locally on the surface of the moon at an early stage of lunar history.