Discovery of coesite and stishovite in eucrite - PubMed (original) (raw)

Discovery of coesite and stishovite in eucrite

Masaaki Miyahara et al. Proc Natl Acad Sci U S A. 2014.

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.

Keywords: meteoroid impact; shock metamorphism.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

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.

Fig. 2.

Fig. 2.

TEM images of silica grain having a network-like texture (Fig. 1_B_). (A) High-angle annular dark-field image of the network-like texture. The network-like texture is a fine-grained granular coesite grain assemblage. A small amount of lamellar stishovite accompanies some network-like texture. (B) TEM image of the outlined section in A. Quartz crystals accompany most of the coesite grain assemblage. Coe, coesite; Fe, metallic iron; Qtz, quartz; Si-gla, silica glass.

Fig. 3.

Fig. 3.

TEM images of silica grain having a lamellae-like texture (Fig. 1_D_). (A) High-angle annular dark-field image of the lamellae-like texture, consisting of stishovite and silica glass. (B) TEM image of the outlined section in A. (C) Selected area electron diffraction pattern corresponding to stishovite. Si-gla, silica glass; Sti, stishovite.

Similar articles

Cited by

References

    1. Yamaguchi A, et al. A new source of basaltic meteorites inferred from Northwest Africa 011. Science. 2002;296(5566):334–336. - PubMed
    1. Wasson JT. Vesta and extensively melted asteroids: Why HED meteorites are probably not from Vesta. Earth Planet Sci Lett. 2013;381:138–146.
    1. Binzel RP, Xu S. Chips off of asteroid 4 Vesta: Evidence for the parent body of basaltic achondrite meteorites. Science. 1993;260(5105):186–191. - PubMed
    1. Marzari F, et al. Origin and evolution of the Vesta asteroid family. Astron Astrophys. 1996;316:248–262.
    1. Burbine TH, et al. Vesta, Vestoids, and the howardite, eucrite, diogenite group: Relationships and the origin of spectral differences. Meteorit Planet Sci. 2001;36(6):761–781.

Publication types

LinkOut - more resources