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 highpressure 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. shock metamorphism | meteoroid impact H owardite-eucrite-diogenite meteorites (HEDs) are the largest group among the achondrites. Although the origin of HEDs is still under debate (1, 2), the similarities between the reflectance spectra of HEDs and the spectra of one of the largest asteroids in the asteroid belt 4 Vesta and dynamic considerations indicate that HEDs originated from 4 Vesta (3-5). The Dawn mission supports this prediction. It has been revealed that many craters exist on 4 Vesta (6, 7), which suggests heavy meteoroid bombardment. The existence of a high-pressure polymorph in a shocked meteorite provides clear evidence for a dynamic event on its parent body (8). Some recent studies propose that 4 Vesta, similar to the Moon, might have suffered from late heavy bombardment (9-11). However, to date, no high-pressure polymorph has been found in HEDs. We now report clear evidence of highpressure polymorphs of silica, coesite, and stishovite from eucrite.We envisaged that some eucrites might contain high-pressure polymorphs because it is expected that the surface of 4 Vesta consists mainly of eucrite. We obtained one of the eucrites, Béréba, to clarify a dynamic event occurring on 4 Vesta, using the high-pressure mineral inventory. The Béréba sample used in this study has many shock-induced melt (hereafter referred to as shock melt) veins (Fig. 1A), implying that it was heavily shocked. Major constituent minerals in the host rock of Béréba are low-Ca pyroxene (Fs 59-63 En 34-37 Wo 2-3 ), augite (Fs 25-32 En 29-31 Wo 38-44 ), plagioclase (An 86-92 Ab 7-14 Or 0-1 ), silica, minor kamacite, ilmenite, chromite, and Ca-phosphate. Most of the low-Ca pyroxene has exsolution lamellae of augite. Plagioclase now transformed into maskelynite partly and/or completely. Flow-like textures appear in some maskelyn...

Microscopic evidence of stishovite generated in low‐pressure shock experiments on porous sandstone: Constraints on its genesis

“…Such a situation was also observed for small natural shock events (Miyahara et al. ; Kaneko et al. ).…”

“…In contrast, there are several examples of melt crystallization of coesite described from natural impacts (e.g., eucrite meteorite—Miyahara et al. ; Ries crater—Fazio et al. ) and from static high‐pressure experiments up to 11 GPa (Kanzaki ; Zhang et al.…”

“…On the one hand, there are microscopic investigations on recovered samples from natural impacts pointing to a solid–solid phase transitions toward stishovite, for example, a linear assemblage of stishovite crystals with similar crystal orientations embedded in an amorphous silica matrix was recently observed in eucrite Béréba (Miyahara et al. ) and in Lunar breccia 15299 (Kaneko et al. ).…”

“…Transmission electron microscopy (TEM) is the ultimate method considering lateral resolution for crystal localization, shape, and defect analysis combined with corresponding diffraction information. On the one hand, there are microscopic investigations on recovered samples from natural impacts pointing to a solid-solid phase transitions toward stishovite, for example, a linear assemblage of stishovite crystals with similar crystal orientations embedded in an amorphous silica matrix was recently observed in eucrite B er eba (Miyahara et al 2014) and in Lunar breccia 15299 (Kaneko et al 2015). In addition, crystal orientation relationships between cristobalite and stishovite embedded in a silica glass were reported in the Martian meteorite Shergotty and Lunar meteorite NWA 4734 (Miyahara et al 2013).…”

“…On the other hand, there are also examples pointing to a melt crystallization mechanism. Round-shaped and/or polycrystalline stishovite with nonpreferred orientation embedded in a glass within melt veins or pockets was observed in Coconino sandstone from Meteor Crater, Arizona (Kieffer et al 1976a) and in the Martian meteorites Zagami Poirier 2000a, 2000b), Shergotty (Beck et al 2004;El Goresy et al 2004), Tissint (Walton et al 2014), in suevite from the Ries crater, Germany (St€ ahle et al 2008), in Lunar breccia 15299 (Kaneko et al 2015), and in eucrite B er eba (Miyahara et al 2014).…”