Discovery of Coesite and Stishovite in Eucrite

Discovery of coesite and stishovite in eucrite Masaaki Miyaharaa,b,1, Eiji Ohtania,c, Akira Yamaguchid, Shin Ozawaa,d, Takeshi Sakaia,e, and Naohisa Hiraof aInstitute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan; bDepartment of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan; cV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090, Russia; dNational Institute of Polar Research, Tokyo 190-8518, Japan; eGeodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan; and fJapan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan Edited by Susan W. Kieffer, University of Illinois at Urbana–Champaign, Urbana, IL, and approved June 25, 2014 (received for review March 5, 2014) Howardite–eucrite–diogenite meteorites (HEDs) probably origi- measured by electron microprobe analysis (Table S1). Raman nated from the asteroid 4 Vesta. We investigated one eucrite, spectroscopy analyses indicate that the original silica grains are Béréba, to clarify a dynamic event that occurred on 4 Vesta using quartz and cristobalite. Representative Raman spectra are shown in a shock-induced high-pressure polymorph. We discovered high- Fig. S1. Some silica grains adjacent to or near the shock-melt veins pressure polymorphs of silica, coesite, and stishovite originating have network-like textures (Fig. 1B). Raman spectroscopy analyses from quartz and/or cristobalite in and around the shock-melt veins further indicate that the network-like textures include the high- of Béréba. Lamellar stishovite formed in silica grains through a pressure polymorphs of silica, coesite, and stishovite, along with solid-state phase transition. A network-like rupture was formed quartz. Transmission electron microscopy (TEM) images indicate and melting took place along the rupture in the silica grains. Nano- that the network-like texture consists of a fine-grained granular sized granular coesite grains crystallized from the silica melt. coesite assemblage (Fig. 2) and minor lamellar stishovite. Amor- Based on shock-induced high-pressure polymorphs, the estimated ∼ ∼ phous (or poorly crystallized) silica exists between coesite and shock-pressure condition ranged from 8to 13 GPa. Considering stishovite grains. Lamellae-like textures are observed in some silica radiometric ages and shock features, the dynamic event that led to grains adjacent to or near the shock-melt veins (Fig. 1 C and D), the formation of coesite and stishovite occurred ca. 4.1 Ga ago, similar to a transition texture from quartz (or cristobalite) to which corresponds to the late heavy bombardment period (ca. 3.8– stishovite (13, 14). TEM images indicate that the silica grains with 4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the lamellae-like texture include lamellar stishovite (Fig. 3). We also investigated pyroxene and plagioclase in and around the shock-melt origin of HEDs is thought to be related to dynamic events that EARTH, ATMOSPHERIC, formed the basins ca. 1.0 Ga ago, our findings are at variance with veins, but their high-pressure polymorphs (e.g., majorite, akimo- AND PLANETARY SCIENCES that idea. toite, jadeite, and lingunite) were not detected. High-pressure polymorphs of silica have been found in lunar shock metamorphism | meteoroid impact meteorites, Martian meteorites, and carbonaceous chondrite (14–17), and in terrestrial impacted rocks (13, 18). Coesite is thermodynamically stable above ∼2.5 GPa (19). Stishovite can be owardite–eucrite–diogenite meteorites (HEDs) are the larg- Hest group among the achondrites. Although the origin of easily synthesized in shock experiments (20, 21). On the other HEDs is still under debate (1, 2), the similarities between the hand, the formation of coesite is not easily achieved in a dynamic reflectance spectra of HEDs and the spectra of one of the largest process. This is because phase transformation from quartz to – asteroids in the asteroid belt 4 Vesta and dynamic considerations coesite is sluggish because of a high kinetic barrier (22 24). indicate that HEDs originated from 4 Vesta (3–5). The Dawn The formation mechanism of shock-induced coesite has been i mission supports this prediction. It has been revealed that many explained as follows ( ): crystallization in the solid state from ii craters exist on 4 Vesta (6, 7), which suggests heavy meteoroid vitrified silica, or ( ) crystallization from silica melt (13, 25). bombardment. The existence of a high-pressure polymorph in Nanosized coesite grain assemblages accompanying minor quartz a shocked meteorite provides clear evidence for a dynamic event and/or stishovite grains occur in the network-like textures of on its parent body (8). Some recent studies propose that 4 Vesta, silica grains in Béréba (Figs. 1B and 2). Coesite that occurs in similar to the Moon, might have suffered from late heavy bom- lunar meteorites is also a nanosized crystal assemblage embed- bardment (9–11). However, to date, no high-pressure polymorph ded in silica glass (17), which is suggestive of a quench crystal has been found in HEDs. We now report clear evidence of high- from silica melt. When quartz is deformed in a piston cylinder at pressure polymorphs of silica, coesite, and stishovite from eucrite. We envisaged that some eucrites might contain high-pressure Significance polymorphs because it is expected that the surface of 4 Vesta consists mainly of eucrite. We obtained one of the eucrites, Quartz and/or cristobalite in eucrite were transformed into Béréba, to clarify a dynamic event occurring on 4 Vesta, using denser minerals, coesite and stishovite, under transient high- the high-pressure mineral inventory. The Béréba sample used in pressure and high-temperature conditions. Coesite and this study has many shock-induced melt (hereafter referred to as stishovite probably formed simultaneously under pressures of A shock melt) veins (Fig. 1 ), implying that it was heavily shocked. similar magnitudes but under different temperature conditions. Major constituent minerals in the host rock of Béréba are low-Ca The expected age of the dynamic event that formed coesite pyroxene (Fs59–63En34–37Wo2–3), augite (Fs25–32En29–31Wo38–44), and stishovite is ca. 4.1 Ga ago, which is inconsistent with the plagioclase (An86–92Ab7–14Or0–1), silica, minor kamacite, ilmen- predicted formation age (ca. 1.0 Ga) of the impact basins on ite, chromite, and Ca-phosphate. Most of the low-Ca pyroxene 4 Vesta. has exsolution lamellae of augite. Plagioclase now transformed into maskelynite partly and/or completely. Flow-like textures Author contributions: M.M. and E.O. designed research; M.M., E.O., A.Y., S.O., T.S., and appear in some maskelynite. Mixing between plagioclase and N.H. performed research; M.M. analyzed data; and M.M., E.O., and A.Y. wrote the paper. pyroxene occurs in the flow-like textures, suggesting that the The authors declare no conflict of interest. feldspar was once melt quenched to maskelynite (12). This article is a PNAS Direct Submission. In this study, the focus of our interest was silica. The silica 1To whom correspondence should be addressed. Email: [email protected]. ∼ μ grain is up to 300 m across. The chemical compositions of the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. silica grains (especially the impurities present, such as Al) were 1073/pnas.1404247111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1404247111 PNAS Early Edition | 1of4 Downloaded by guest on September 24, 2021 Fig. 1. Back-scattered electron images of silica grains in Béréba. (A) Low-magnification image of a shock-melt vein. Quartz (and/or cristobalite) grains exist in and around the shock-melt veins. Feldspar partially transforms to maskelynite. (B) Silica grains with a network-like texture. Coesite, stishovite, and silica glass (+ minor quartz) coexist in the silica grain. (C) Silica grains with a lamellae-like texture. Stishovite and silica glass coexist in silica grains. (D) High-magnification image of the outlined section in C. Coe, coesite; Fd, feldspar; Pyx, pyroxene; Qtz, quartz; Si-gla, silica glass; Sti, stishovite. 2.7–3.0 GPa under ambient temperature conditions, coesite that stishovite is very sensitive to heating. Some stishovite might forms along melted ruptures in the deformed quartz (26). become amorphous during adiabatic decompression. The network-like texture of Béréba indicates a flow-like tex- The pressure conditions in a silica grain of Béréba would be ture; it would be a mixture of silica and plagioclase because small homogeneous; whereas, the temperature conditions are hetero- amounts of calcium and aluminum, as well as silicon, are con- geneous because some portions are heated beyond the melting tained in the texture. When a silica grain was shocked, it was temperature and melting takes place along the fractures. The ruptured and, simultaneously, melting took place because of Clapeyron slope between coesite and stishovite, deduced from friction along the ruptures. The crystallization of coesite from laboratory high-pressure static experiments, is positive (29). silica melt would have a lower kinetic barrier than a solid–solid Accordingly, both coesite and stishovite probably formed under phase transformation. Nanosized coesite grain assemblages similar pressure conditions,

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