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Jadeite in Shocked Meteorites and Its Textural Variations

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Jadeite in Shocked Meteorites and Its Textural Variations Journal of Mineralogical and Petrological Sciences, Volume 112, page 247–255, 2017 REVIEW Jadeite in shocked meteorites and its textural variations Eiji OHTANI*,**, Shin OZAWA* and Masaaki MIYAHARA*** *Department of Earth Science, 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 ***Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi–Hiroshima 739–8526, Japan Jadeite occurs as the shocked product of albite feldspar in shocked meteorites, and is one of the most common high–pressure polymorphs in shock–melt veins of meteorites. The characteristic textures of jadeite in shocked ordinary chondrites show that some of jadeite crystals were formed from originally albite feldspar by a solid– state transformation and some were crystallized from a shock–induced albite melt. Based on these textures of jadeite together with the other high–pressure mineral assemblages and their crystallization kinetics, we can estimate the impact conditions such as impact velocity and parent–body size. Keywords: Jadeite, Ordinary chondrite, Shocked meteorite, Crystallization, Solid–state transformation INTRODUCTION polymorph. The phase diagram of albite based on the stat- ic high–pressure and high–temperature experiments indi- Shocked meteorites contain high–pressure polymorphs of cates that albite transforms to NaAlSi3O8 with hollandite– constituent minerals, such as high–pressure polymorphs structure (lingunite) or CaFe2O4–type NaAlSiO4 + stisho- of olivine, pyroxene, feldspar, and silica minerals. Such vite subsequent to jadeite + SiO2 assemblages with in- high–pressure polymorphs were formed by collisions in creasing pressure and temperature (e.g., Holland, 1980; the early solar system and the impact events on the sur- Yagi et al., 1994; Liu, 2006; Tutti, 2007). Jadeite occurs faces of Mars and Moon. Recent discoveries of new as the shocked product of albite feldspar, and is one of the high–pressure polymorphs (e.g., El Goresy et al., 2000; most common high–pressure polymorphs in shocked me- Hollister et al., 2014; Bindi et al., 2017; Litasov and teorites. Jadeite and tissintite, Ca–bearing isostructural Podgornykh, 2017) drastically increased the number of phase of jadeite, have been discovered in six different meteorite types containing high–pressure polymorphs types of meteorites with the compositions close to basalt. since early 2000s. Figure 1 shows the classification tree Jadeite was reported in H, L, and LL ordinary chondrites of meteorites. The meteorite groups containing high– (e.g., Ohtani et al., 2004; Miyahara et al., 2013; Ozawa et pressure polymorphs are shown as the shaded boxes in al., 2014) and CB group carbonaceous chondrite (Miya- this figure. Now we can see 13 groups of meteorites con- hara et al., 2015). Whereas, shergottite and eucrite contain tain high–pressure polymorphs. tissintite which is the isostructural phase of jadeite con- Albite is one of the major constituents of ordinary taining Ca in its structure (Ma et al., 2015; Pang et al., chondrites. Some albite grains become maskelynite in 2016). Identification of jadeite in meteorites has been shocked ordinary chondrites. Albite grains entrained in made by using micro–Raman spectroscopy. The typical or adjacent to the shock–melt veins of these meteorites Raman spectrum of jadeite in the shock melt vein of Che- experienced both high–pressure and high–temperature lyabinsk LL5 ordinary chondrite measured with a micro– conditions due to localized frictional heating during the Raman spectrometer (JASCO NRS–5100) is shown in shock events. Accordingly, albite entrained in or adjacent Figure 2. to the shock–melt veins transformed into its high–pressure Two distinct textures, i.e., the solid state transforma- tion and crystallization from the melt, have been ob- doi:10.2465/jmps.170329 served as the high pressure polymorphs of minerals in- E. Ohtani, [email protected] Corresponding author cluding jadeite, olivine, low–Ca pyroxene, and garnet. 248 E. Ohtani, S. Ozawa and M. Miyahara Figure 1. Meteorite classification tree. That presented by Weisberg et al. (2006) was modified in this figure. Meteorite types including high– pressure polymorphs are shadowed. Meteorite types including jadeite (or tissintite) are highlighted with a dark shadow (CB, H, L, LL, SHE, EUC). URE, ureilite; BRA, brachinite; ACA, acapulcoite; LOD, lodranite; WIN, winonaite; SHE, shergottite; NAK, nakhlite; CHA, chas- signite; OPX, orthopyroxenite; ANG, angrite; AUB, aubrite; EUC, eucrite; DIO, diogenite; HOW, howardite; MES, mesosiderite; MG PAL, main–group pallasite; ES PAL, Eagle Station pallasite; PP PAL, pyroxene pallasite. The contrasting textures are observed in olivine in differ- ning electron microscopy (SEM), transmission electron ent L6 chondirites (Ohtani et al., 2004, 2006). microscopy (TEM) and X–ray diffraction (XRD). Differ- Olivine crystals existing along the shock–melt veins ent textures of formation of jadeite with or without silica in Yamato (Y)–791384 L6 chondrite contain ringwoodite and lingunite from albite feldspar has been reported in with a lamellar texture. The lamellar texture indicates that many previous studies. These phase assemblages record ringwoodite was formed by the solid–state transformation the pressure and temperature conditions during the shock with a mechanism of the coherent nucleation and suc- event occurred on its parent–body. Here, we review the ceeding incoherent nucleation (Ohtani et al., 2004; Miya- recent works on textures of jadeite in several shocked hara et al., 2010). On the other hand, wadsleyite–ring- meteorites, and discuss the pressure and temperature con- woodite assemblage occurred in the shock–melt veins ditions during the shock events for formation of jadeite of Allan hills (ALH) 78003 and Peace River L6 ordinary with different textures. chondrites, indicating overgrowth in the melt at high– pressure (Ohtani et al., 2006) or fractional crystallization TEXTURAL VARIATIONS OF JADEITE IN from the melt (Miyahara et al., 2008, 2009). SHOCKED METEORITES As mentioned above, jadeite is commonly identified in many kinds of shocked meteorites as a high–pressure Solid–state transformation polymorph formed from albite feldspar. The occurrence of jadeite in shocked meteorites has been investigated in Kimura et al. (2000) confirmed the existence of jadeite in detail by using laser micro–Raman spectroscopy, scan- albite feldspar grains entrained in or adjacent to the shock– Jadeite in shocked meteorites 249 melt veins of Y–74445 L6 ordinary chondrite (Ozawa et al., 2009). Original albite feldspar (Ab83–85An10Or5–7) grains entrained in the shock–melt veins of Y–74445 transformed into jadeite and lingunite without any crys- talline silica phases. The lamellar intergrowth of jadeite and lingunite indicates these phases were formed by the solid state transformation. Jadeite crystallite assemblages contain many ‘particle–like’ materials as shown in Figure 3B. Ozawa et al. (2009) described that the ‘particle–like’ material was silica–rich amorphous material, and pro- posed that original albite feldspar transformed to jadeite and the amorphous material. Similar jadeite crystallite assemblages were also observed in the albite feldspar (Ab83–85An10Or5–7) grains entrained in the shock–melt veins of Sahara 98222 L6 ordinary chondrite (Ozawa et al., 2009). Numerous particle–like or stinger–like materi- als occurred coexisting with jadeite in the albite feldspar grains of Sahara 98222. Their typical textures are shown in Figure 4. Ozawa et al. (2009) suggested that the parti- Figure 2. A typical Raman spectrum of jadeite from Chelyabinsk – – LL5 ordinary chondrite (Ozawa et al., 2014) measured with a cle like and stringer like materials may be composed of micro–Raman spectrometer (JASCO NRS–5100). Ol, Raman silica–rich amorphous materials. Further detail investiga- peaks of neighboring olivine. tion on jadeite in shocked ordinary chondrites was con- ducted using focused ion beam (FIB)–assisted TEM and XRD patterns together with FE–SEM and Raman spec- melt veins of Yamato (Y)–75100 H6 ordinary chondrite troscopy. Miyahara et al. (2013) investigated jadeite oc- using a laser micro–Raman spectroscopy for the first time. curring in the albite feldspar (Ab84–86An9–10Or5–7 in Y– When jadeite is formed by the decomposition reaction of 791384; Ab80–81An13–14Or5–6 in Y–75100) grains entrain- albite, silica phase should be accompanied with jadeite ed in or adjacent to the shock–melt veins of Y–791384 L6 based on the composition of albite; i.e., albite (NaAlSi3 and Y–75100 H6 ordinary chondrites. Synchrotron XRD O8) → jadeite (NaAlSi2O6) + silica (SiO2) (e.g., Birch patterns and TEM observations revealed that the albite and LeCompte, 1960; Boyd and England, 1963). Y– feldspar grains contain jadeite and residual amorphous 75100 also contains wadsleyite, lingunite and majorite– (or poorly–crystallized) materials. pyrope solid–solution as high–pressure polymorphs, indi- Kubo et al. (2010) conducted in situ X–ray diffrac- cating that the peak shock pressure condition exceeded ~ tion measurements of two kinds of feldspar (albite and 15 GPa based on the stability fields of the high–pressure labradorite) under the conditions of high–pressure and assemblages (Kimura et al., 2000). Considering the shock high–temperature below the liquidus temperature of the pressure conditions recorded in the shock–melt
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