51st Lunar and Planetary Science Conference (2020) 1803.pdf HEATING EXPERIMENTS OF MASKELYNITE IN ZAGAMI, ELEPHANT MORAINE A79001 AND ALLAN HILLS 77005: IMPLICATIONS FOR THE EFFECT OF SHOCK HEATING. R. Shikina1,2 and T. Mikouchi1,2, 1Department of Earth and Planet. Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan (
[email protected]), 2The University Museum, The University of Tokyo, Hongo, Tokyo 113-0033, Japan. Introduction: Plagioclase is one of the important along the cracks and at edges of the original grains. constituents of extraterrestrial materials and is sensi- Although maskelynites appear to have a smooth sur- tive to the shock metamorphism. Maskelynite is under- face through optical microscopy, the recrystallized stood as a diaplectic plagioclase glass that has been plagioclase shows a dirty devitrified texture. Electron transformed from plagioclase in solid-state transfor- microprobe analysis (shown in Fig. 2) revealed that Na mation by strong impact and is commonly found in and K are reduced, but Ca is enriched in the recrystal- Martian meteorites [1]. Maskelynite is known to be lized plagioclase of the samples showing partial recrys- easily converted to crystalline plagioclase by reheating tallization (900 °C for 8 and 24 hours and 1000 °C for and the degree of recrystallization depends upon shock 1 hour). In the maskelynite grains heated at 900 °C for pressure [2]. Such plagioclase recrystallizations is 168 hours, small glass regions enriched in Na and K found in meteorites. For example, the lunar meteorite are present (Na2O: ~6.5 wt%, K2O: ~2.5 wt%), sug- Y-793169 shows a recrystallized polycrystalline plagi- gesting that Na and K concentrate in areas where re- oclase texture by a reheating event after the formation crystallization did not start yet.