Lunar meteorite Yamato-791197: Petrography, shock history and chemical composition
Antarctic meteorite Yamato-791197 (Y-791197) is a polymict regolith breccia from the lunar highlands. It is composed of feldspathic lithic fragments, glasses and mineral fragments in a densely compacted, fine-grained, partly glassy matrix. Dominant lithologies are granulitic anorthosites, granulitic breccias, recrystallized fragmental breccias, microporphyritic and subophitic crystalline melt breccias, glass beads and irregularly shaped glassy particles. Some glasses are partly or totally devitrified. All mineral clasts are shocked, most plagioclase clasts are recrystallized. The shock effects in mineral and lithic clasts indicate pressures below 25GPa except for rare maskelynite and the shock-produced glassy particles. There is no indication that the meteorite as a whole has been shocked to more than 20GPa, consequently the shock pressures during the ejection event were less than 20GPa. Despite these low shock pressures, the breccia may have formed by shock-lithification caused by the ejection event. Chemical analyses of the bulk sample and of individual components indicate a source area free of KREEP, probably on the lunar farside. Differences in chemical composition (major and minor elements) may exist between the lunar frontside and the lunar farside. The content of Ga and Zn is remarkably high but heterogeneously distributed and may speak in favor of a moon-wide redistribution of volatile elements. The noble gas contents of Y-791197 are typical of lunar regolith and yield an exposure age of 450Ma. Y-791197 has a higher content in REE and noble gases and is more ferroan than lunar meteorite ALHA81005. Both differ compositionally from other lunar samples and despite their differences may be related to the same impact event.