title: Discovery of coesite and stishovite in eucrite.
abstract: Howardite-eucrite-diogenite meteorites (HEDs) probably originated from the asteroid 4 Vesta. We investigated one eucrite, Béréba, to clarify a dynamic event that occurred on 4 Vesta using a shock-induced high-pressure polymorph. We discovered high-pressure polymorphs of silica, coesite, and stishovite originating from quartz and/or cristobalite in and around the shock-melt veins of Béréba. Lamellar stishovite formed in silica grains through a solid-state phase transition. A network-like rupture was formed and melting took place along the rupture in the silica grains. Nanosized granular coesite grains crystallized from the silica melt. Based on shock-induced high-pressure polymorphs, the estimated shock-pressure condition ranged from ∼8 to ∼13 GPa. Considering radiometric ages and shock features, the dynamic event that led to the formation of coesite and stishovite occurred ca. 4.1 Ga ago, which corresponds to the late heavy bombardment period (ca. 3.8-4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the origin of HEDs is thought to be related to dynamic events that formed the basins ca. 1.0 Ga ago, our findings are at variance with that idea.
authors.affiliation: Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan; miyahara@hiroshima-u.ac.jp.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Miyahara
authors.affiliation: Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090, Russia;
authors.forename: Eiji
authors.initials: E
authors.lastname: Ohtani
authors.affiliation: National Institute of Polar Research, Tokyo 190-8518, Japan;
authors.forename: Akira
authors.initials: A
authors.lastname: Yamaguchi
authors.affiliation: Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;National Institute of Polar Research, Tokyo 190-8518, Japan;
authors.forename: Shin
authors.initials: S
authors.lastname: Ozawa
authors.affiliation: Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan; and.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Sakai
authors.affiliation: Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan.
authors.forename: Naohisa
authors.initials: N
authors.lastname: Hirao
keywords: meteoroid impact; shock metamorphism