[P-PS22_1AM2] Planetary processes from meteorites and experimental works

[PPS22-10] Discovery of stishovite in an Apollo 15 sample and impact record on the Moon

Shohei KANEKO¹, *Eiji OHTANI¹, Masaaki MIYAHARA², Shin OZAWA¹, Tomoko ARAI³ (1.Tohoku University, 2.Hiroshima University, 3.Chiba Institute of Technology)

Keywords:Stishvite, Apollo mission, Impact, High pressure and temperature, Lunar sample

Thick regolith layers and many craters on the Moon indicate that the Moon has been heavily bombarded after the lunar formation. Short time intervals of high-pressure and high-temperature occurred on the lunar surface during the collision of asteroids on the Moon, and the constituent minerals of the Moon and asteroids transformed into high-pressure polymorphs during the high-pressure and high-temperature conditions. Although many brecciated lunar rocks have been recovered by the Apollo missions, any high-pressure polymorph has not been observed in Apollo samples so far. Silica is one of constituent minerals of terrestrial planets and asteroid. We investigated a lunar regolith collected by the Apollo 15 mission with a special interest on silica, because high-pressure polymorphs of silica are recently reported from shocked lunar meteorites (Ohtani et al., 2011; Miyahara et al., 2013). Here, we show stark evidence for stishovite from a sample collected by the Apollo 15 mission. X-ray diffraction analysis and transmission electron microscopic observations clearly confirmed the existence of a high-pressure polymorph of silica, stishovite, in the Apollo sample, which suggests that the lunar legolith preserves records of early shock events. Considering radio-isotope ages, lithologies, and shock features, stishovite was formed by an impact event in the near side Moon ca. 3.8-4.1 Ga ago.