A Rb-Sr age of glassy material from a lunar meteorite and its origin on the moon

Abstract

The impact of a planetesimal with a planet produces craters. Part ia l or complete melting may be induced on and around the collision spot. Chronological studies of these re-melted samples should give us the ages of the impacts and informat ion about crater format ion on planets and satellites. Here we report for the first time a 87Rb87Sr study on an impact-melted sample (Yamato-82192 SubNo.63B), one of the lunar meteorites, the moon rocks having been unexpectedly recovered on the earth. The chip, Y-82192 SubNo.63B, was collected near the Yamato Mountains, Antarct ica [1]. The lunar meteorites, including Y-82192, are considered to have derived from lunar highland, based on the petrological and chemical studies [2, 3]. The chip, Y-82192 SubNo.63B, shows a glassy feature distinct from other lunar meteori te samples characterized as anorthosi te breccia. This glassy l i thology is considered to be produced by impact melting. A small part (5 mg) of the sample was used for analysis on a whole rock sample, other parts ( ~ 60 mg) were crushed and separated into several fractions with heavy liquids (methylene iodide and bromoform), and Rb-Sr analyses were performed on each fraction. (The fraction suspended in acetone is called "suspension" here). Sr isotope ratios were measured with a VG 354 mass spectrometer and the abundances of Sr and Rb were analyzed by the isotope dilut ion method, using a mass spectrometer (JEOL JMS-05RB). The results are shown in Table 1. The errors for abundances are within 0.5 %. Based on the analyses of the major element compositions by E P M A (JEOL JXA-733), the fractions with densities lower than 3.25 g cm -3 are composed chiefly of Ca-rich plagioclase or Ca,Al-r ich glass. The fraction with the highest density (higher than 3.25 g cm -3) is composed of olivine with a small amount of pyroxene. As reported previously [3], the REE abundances in this melted sample remained unchanged irrespective of melting. On the other hand, as shown in Fig. 1, the Rb-Sr system was significantly disturbed by the melting event, although the whole rock sample of Y82192, SubNo.63B, lies near a 3.95 x 109a line defined by other lunar meteorites (see solid line in Fig. 1 ; the da ta of the solid line are from [3]). Among the separated fractions plus the whole rock under consideration, five (four fractions with densities lower than 3.25 and the whole rock sample) fall nearly on a line and the other two fractions (suspension and a fraction with density higher than 3.25) lie at distinct positions. Concerning the high melting tem-