Shocked plagioclase in the lunar meteorites Yamato-793169 and Asuka-881757: Implications for their shock and thermal histories

Yamato (Y)-793169 and Asuka (A)-881757 are unbrecciated lunar mare meteorites that contain shocked plagioclase as a main constituent phase. Plagioclase in Y-793169 is composed of fine-grained microlitic grains that preserve the overall optical orientation of the original grain. Small vesicles are also observed within plagioclase grains. Plagioclase shows chemical zoning from the An-rich core (An_) to the An-poor rim (An_). These observations suggest that plagioclase in Y-793169 recrystallized from a diaplectic glass by thermal annealing as the heating experiment of a diaplectic plagioclase suggests. The thermal annealing event might occur sometime later than 750Ma that has been derived from the K-Ar study of plagioclase. A-881757 plagioclase has completely transformed into an isotropic glass similar to "maskelynite" in Martian meteorites. Chemical zoning of plagioclase is well preserved, showing a systematic change of composition from the An-rich core (An_) to the slightly An-poor rim (An_). Plagioclase glass partly converts to crystalline (optically single crystal) plagioclase near the fusion crust due to reheating during the atmospheric entry. All of these observations suggest that plagioclase glass in A-881757 is probably a diaplectic glass. Unlike Y-793169,plagioclase glass in A-881757 has not been modified by later thermal events, which is consistent with the old ^Ar-^Ar age (3.8Ga) of A-881757 plagioclase glass. Thus, Y-793169 and A-881757 contained (A-881757 still contains) a diaplectic plagioclase glass that was shock-induced perhaps during the late heavy bombardment of the Moon as their old crystallization ages (3.8-3.9Ga) indicate