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Home , Coesite, Eucrite, Seifertite, Stishovite, Suevite

Discovery of and 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) –eucrite– (HEDs) probably origi- measured by electron microprobe analysis (Table S1). Raman nated from the . 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 and . 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 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 origin of HEDs is thought to be related to dynamic events that investigated and in and around the shock-melt formed the basins ca. 1.0 Ga ago, our findings are at variance with veins, but their high-pressure polymorphs (e.g., , akimo- that idea. toite, jadeite, and lingunite) were not detected. High-pressure polymorphs of silica have been found in lunar shock | impact meteorites, Martian meteorites, and carbonaceous (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 . Although the origin of easily synthesized in shock experiments (20, 21). On the other hand, the formation of coesite is not easily achieved in a dynamic EARTH, ATMOSPHERIC,

HEDs is still under debate (1, 2), the similarities between the AND PLANETARY SCIENCES reflectance spectra of HEDs and the spectra of one of the largest process. This is because phase transformation from quartz to – 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 provides clear evidence for a dynamic event and/or stishovite grains occur in the network-like textures of on its (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 , coesite and stishovite, under transient high- the high-pressure 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 , 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 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. 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 | July 29, 2014 | vol. 111 | no. 30 | 10939–10942 Downloaded by guest on October 2, 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 , 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, but coesite crystallized under a would form from the melted silica. On the other hand, the silica higher temperature conditions from the silica melt, whereas grain around the melted silica was not heated, by friction, beyond stishovite formed under lower temperature conditions in a solid its melting temperature. The unmelted silica portions indicate state. The coexistence of coesite and stishovite would be due to a smooth surface or lamellae-like texture, and consist of amor- the significant temperature gradient in the silica grain. phous silica or stishovite. Static high-pressure synthetic experi- The dynamic event that formed both coesite and stishovite is ments indicate that quartz and/or cristobalite transform to the most intense impact incident recorded in Béréba. We could stishovite (or ), or become amorphous, through a pres- constrain a pressure condition recorded in Béréba using high- sure-induced phase transformation at high pressure but under pressure polymorphs. It is expected that the stable pressure field low-temperature conditions (27, 28). Stishovite in Béréba is of stishovite depends on the alumina content, based on high- formed through a solid–solid phase transition from original pressure synthetic experiments using alumina-bearing silica (30). quartz/cristobalite. TEM observations with electron-beam irra- However, the silica present in Béréba contains hardly any alu- diation indicate that stishovite vitrified immediately, implying minum (<0.2 wt% as Al2O3; Table S1). We adopted a phase

10940 | www.pnas.org/cgi/doi/10.1073/pnas.1404247111 Miyahara et al. Downloaded by guest on October 2, 2021 Fig. 2. TEM images of silica grain having a network-like texture (Fig. 1B). (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)TEMimageofthe outlined section in A. Quartz crystals accompany most of the coesite grain assemblage. Coe, coesite; Fe, metallic iron; Qtz, quartz; Si-gla, silica glass.

diagram deduced from static high-pressure synthetic experi- the relatively younger isotope age (ca. 4.1 Ga) corresponds to ments using pure silica to estimate a shock-pressure condition. a dynamic event that formed the shock-melt veins, including Considering the phase diagram, the coexistence of coesite and high-pressure polymorphs, and induced melting and quenching of stishovite suggests that the pressure condition recorded in Béréba plagioclase to maskelynite glass. It is expected that heavy meteorite would range from ∼8to∼13 GPa (29, 31). bombardment occurred on the Moon around 3.8–4.1 Ga ago (33). Bulk-rock Pb–Pb isotope age indicates not only an older age The existence of high-pressure polymorphs in Béréba may support (4.521 ± 0.0004 Ga) but also a younger age (ca. 4.1 Ga) (32). the idea that a parent body of HEDs also suffered from such a When shock-induced glass veinlets in the bulk rock are removed cataclysm, as deduced from 40Ar–39Ar isotope age distributions from a crushed bulk-rock sample, the relatively younger age is (9–11). EARTH, ATMOSPHERIC,

removed, implying that the relatively younger isotope ages are Two giant impact basins (Rheasilvia and Veneneia) on 4 Vesta AND PLANETARY SCIENCES closely related to an event that formed the shock-melt vein (32). are depicted by the Dawn mission. Model calculation and crater U–Pb systems are hosted in feldspar (32). Plagioclase in Béréba chronology obtained by the Dawn mission reveal that the giant melted and quenched to maskelynite glass. Induced by a dynamic impact basins would have formed around 1.0 Ga ago (4, 34). event, the U–Pb systems would be disturbed through the melting Fragments were launched from 4 Vesta by the giant impact-basin and quenching of plagioclase to maskelynite glass. Accordingly, formations, and became the Vesta family in the asteroid belt. In

Fig. 3. TEM images of silica grain having a lamellae-like texture (Fig. 1D). (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.

Miyahara et al. PNAS | July 29, 2014 | vol. 111 | no. 30 | 10941 Downloaded by guest on October 2, 2021 addition, some of them fell into the Earth as HEDs. The dy- 15 kV was used. Chemical compositions of minerals were determined using the namic events that formed two giant impact basins are the most wavelength-dispersive procedure with a JEOL JXA-8800M electron microprobe catastrophic events to occur on 4 Vesta since it formed. On the analyzer. Analyses were carried out using an accelerating voltage of 15 kV, – μ other hand, the most intense dynamic event recorded in Béréba a beam current of 10 nA and a defocused beam of 1 10 m. Slices for TEM observations were prepared using a focused ion beam (FIB) is ca. 4.1 Ga ago, which contradicts the launch time of HEDs 40 –39 system, JEOL JEM-9320FIB. JEOL JEM-2010 and JEM-2100F transmission from 4 Vesta. Most resetting timings of Ar Ar isotopic electron microscopes operating at 200 kV were used for conventional TEM ages (9, 10) do not coincide with the HEDs formation model observations and selected area electron diffraction. The JEOL JEM-2100F is deduced from model calculation and crater chorology. As- equipped with a scanning TEM (STEM) mode and a JEOL energy dispersive suming that HEDs originate from 4 Vesta, HEDs would not X-ray spectroscopy (EDS) detector system. The chemical compositions of in- originate from the that formed the Rheasilvia or dividual minerals were obtained by EDS under STEM mode. The compositions Veneneia basins. were corrected using theoretically determined k factors. The unused Béréba samples are stored at Department of Earth and Planetary Systems Science, Methods Graduate School of Science, Hiroshima University, Japan. The Béréba sample studied here was obtained through Bruno Fectay and Carine Bidaut of www.meteorite.fr. A polished Béréba chip sample was prepared for ACKNOWLEDGMENTS. This study was supported by Ministry of Education, Culture, Sports, Science and Technology Grants-in-Aid for Scientific Research the study, and petrological observation was carried out with an optical micro- 22000002 (to E.O.) and 26800277 (to M.M.). This work was also partly sup- scope. The mineralogy was determined using a laser micro-Raman spectrometer, ported by the Ministry of Education and Science of Russian Federation, JASCO NRS-5100. A microscope was used to focus the excitation laser beam Project 14.B25.31.0032. This work was conducted as a part of Tohoku Uni- (green laser; 532 nm). We used a field-emission gun scanning electron micro- versity’s Global Center of Excellence program Global Education and Research scope, JEOL JSM-71010, for fine textural observations. An accelerating voltage of Center for Earth and Planetary Dynamics.

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