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Mg, Fe)Sio3 Glass in the Suizhou Meteorite

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Mg, Fe)Sio3 Glass in the Suizhou Meteorite Meteoritics & Planetary Science 39, Nr 11, 1797–1808 (2004) Abstract available online at http://meteoritics.org A shock-produced (Mg, Fe)SiO3 glass in the Suizhou meteorite Ming CHEN,1* Xiande XIE,1 and Ahmed EL GORESY2 1Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China 2Max-Planck-Institut für Chemie, D-55128 Mainz, Germany *Corresponding author. E-mail: [email protected] (Received 24 February 2004; revision accepted 15 August 2004) Abstract–Ovoid grains consisting of glass of stoichiometric (Mg, Fe)SiO3 composition that is intimately associated with majorite were identified in the shock veins of the Suizhou meteorite. The glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low-Ca pyroxene and majorite. These ovoid grains are surrounded by a fine-grained matrix composed of majorite-pyrope garnet, ringwoodite, magnesiowüstite, metal, and troilite. This study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature, while at the rim region pyroxene grains transformed to majorite due to a higher temperature. After pressure release, perovskite vitrified at post-shock temperature. The existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 GPa. The post-shock temperature in the meteorite could have been above 477 °C. This study indicates that the occurrence of high-pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites. INTRODUCTION also be transformed to amorphous phase or glass at shock- produced high pressure and temperature. Pyroxene glass has Pyroxene (Mg, Fe)SiO3 is a major rock-forming mineral been reported in some shocked meteorites. Price et al. (1979) in the upper mantle of Earth and in stone meteorites. At high- reported on the transmission electron microscopy (TEM) pressure and temperature, pyroxene transforms to high- observations of a small amount of retrograde glass thought to pressure polymorphs including majorite, ilmenite, and have inverted from majorite in the shock veins of the Tenham perovskite. High-pressure polymorphs are considered to be meteorite. In addition, nanometer-sized pyroxene glass was among the major constituents of the Earth’s deep mantle (Liu reported to occur at the rim region of crystalline pyroxene in 1974, 1975, 1976, 1980). These high-pressure polymorphs the Allan Hills (ALH) 84001 meteorite as a result of have been found in the shock veins of chondrites (Smith and incipient shock melting (Bell et al. 1999). Malavergne et al. Mason 1970; Price et al. 1979; Langenhorst et al. 1995; Chen (2001) reported Si-Al-rich glass and augite glass in shocked et al. 1996; Sharp et al. 1997; Tomioka and Fujino 1997), but martian meteorites. Recently, Tomioka and Kimura (2003) not in terrestrial rocks. Majorite frequently occurs in those reported a shock-produced mixture of fine-grained majorite strongly shocked meteorites. However, perovskite was not and CaSiO3-rich glass transformed from diopside in an H widely identified in these meteorites. Up to now, only two chondrite. meteorites were reported to contain very small amounts of In comparison to previously found perovskite, vitrified “perovskite.” Tomioka and Fujino (1997) found that perovskite, and pyroxene glass, here we report a unique crystalline perovskite transformed from orthopyroxene in the occurrence of fine-grained multi-granular (Mg, Fe)SiO3 glass Tenham meteorite. Sharp et al. (1997) found vitrified that intimately coexisted with majorite in the shock veins of perovskite in the shock veins of the Acfer 040 meteorite, the Suizhou meteorite. We found large (Mg, Fe)SiO3 glass which was interpreted as an amorphized perovskite during grains that appear to represent the most extensive pyroxene- pressure release. The natural occurrence of perovskite is not perovskite transformation observed in meteorites. only important for clarifying the phase transformations of pyroxene and the history of pressure and temperature in Analytical Procedures shocked meteorites, but also for understanding the phase transformation processes in the Earth’s mantle. Polished thin sections were prepared from the shock- In addition to high-pressure polymorphs, pyroxene may vein-bearing fragments of the Suizhou meteorite. A Hitachi 1797 © Meteoritical Society, 2004. Printed in USA. 1798 M. Chen et al. S-3500N scanning electron microscope equipped with a Link shock veins (Wang and Li 1990). Petrographic investigations ISIS 300 X-ray energy dispersive spectrometer at the indicated that this meteorite was moderately shock- Guangzhou Institute of Geochemistry, Chinese Academy of metamorphosed and classified as shock stage 3–4 (Xie et al. Sciences, was employed for detailed mineral and textural 2001, 2003; Chen et al. 2003). investigations in backscattered electron mode. The meteorite consists of olivine, pyroxene (mainly low- The quantitative chemical analyses of various phases Ca pyroxene and small amount of Ca-pyroxene), plagioclase, were made with a JXA-8900RL electron microprobe at 15 kV kamacite, taenite, troilite, and accessory minerals (chromite, accelerating voltage and 10 nA sample current at Mainz whitlockite, and apatite) (Xie et al. 2001). Most of these University (Germany). minerals have intact structures, except for plagioclase. Raman spectra of minerals and other solid-state phases Olivine and pyroxene display a weak mosaic texture and were recorded with a Renishaw RM-2000 instrument at the usually contain abundant regular fractures and 3 to 4 sets of Guangzhou Institute of Geochemistry (China). A microscope planar fractures (Fig. 1). Optically, the undulatory extinction was used to focus the excitation beam (Ar+ laser, 514 nm line) is very weak for most olivine and pyroxene grains. About to a 2 µm spot and to collect the Raman signal. 30% of the plagioclase grains have normal optical properties Accumulations lasting for 300 sec were made. The laser which indicate that its crystal structure remains intact power on the samples was limited to <1–2 mW to avoid (Figs. 2a and 2b). Most of the remaining plagioclase grains deterioration of the samples due to laser heating. have a reduced birefringence and contain abundant planar deformation features (PDFs), and some grains display a RESULTS partially isotropic nature (Figs. 2c and 2d). Only the plagioclase grains that are close to the shock veins (at a Petrography of the Suizhou Meteorite distance of <300–400 µm) were transformed to maskelynite, a melted plagioclase glass (Figs. 2e and 2f). The Suizhou meteorite fell in Hubei, China in 1986. A The shock veins were very poorly developed in the total of 270 kg of the meteorite was recovered. This meteorite meteorite. We examined all fragments of the meteorite and was classified as an L6 chondrite and contains a few very thin only found a few very thin black veins of chondritic Fig. 1. A backscattered electron (BSE) image showing a shock vein intersecting the chondritic portion of the Suizhou meteorite. Note that the olivine (Olv), pyroxene (Pyx), and plagioclase (Plg) contain abundant fractures, while maskelynite (Ms) displays little fracturing in the interior; M = Fe-Ni metal; Tr = troilite. A shock-produced glass in Suizhou 1799 Fig. 2. Transmitted light images of a chondritic portion of the Suizhou meteorite: a) a plane polarized image showing the occurrence of plagioclase. Note that these plagioclases are cut by many cracks; b) a crossed polarized image of the same area in (a) showing crystalline plagioclase; Plg = plagioclase; Olv = olivine; M = Fe-Ni metal. 1800 M. Chen et al. Fig. 2. Continued. Transmitted light images of a chondritic portion of the Suizhou meteorite: c) a plane polarized image showing plagioclase with abundant planar deformation features (PDFs); d) a crossed polarized image of the same area in (c) in which lamella-like PDFs can be clearly recognized. This grain has a reduced birefringence and contains locally isotropic area; Plg = plagioclase; Olv = olivine; Pyx = pyroxene; Tr = troilite. A shock-produced glass in Suizhou 1801 Fig. 2. Continued. Transmitted light images of a chondritic portion of the Suizhou meteorite: e) a plane polarized image indicating the occurrence of maskelynite. This image shows smooth and clear interior and no PDFs can be identified; f) a crossed polarized image of the same area in (e) showing completely isotropic maskelynite; Ms = maskelynite; Olv = olivine; Pyx = pyroxene. 1802 M. Chen et al. composition intersecting the rock. No melt pocket of chondritic composition was observed. The shock veins ranging 20–200 µm in width extend for more than 10 to 20 cm in one direction. These thin veins do not form a complex network of veins (Fig. 1). The boundaries of the shock veins are sharp and straight. High-Pressure Minerals in the Shock Veins The shock veins of the Suizhou meteorite contain abundant high-pressure minerals including ringwoodite, majorite, majorite-pyrope garnet and (Na, Ca)AlSi3O8- γ hollandite (Xie et al. 2001), -Ca3(PO4)2-tuite (Xie et al. 2003), and a high-pressure polymorph of chromite with a CaTi2O4-type structure (Chen et al. 2003). In this study, we confirmed the occurrence of each of these high-pressure minerals in the shock veins of the meteorite by using Raman Fig. 3. A backscattered electron (BSE) image of the Suizhou meteorite showing that the shock vein contains coarse-grained,
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