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Ringwoodite Lamellae in Olivine: Clues to Olivine–Ringwoodite Phase Transition Mechanisms in Shocked Meteorites and Subducting Slabs

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Ringwoodite Lamellae in Olivine: Clues to Olivine–Ringwoodite Phase Transition Mechanisms in Shocked Meteorites and Subducting Slabs Ringwoodite lamellae in olivine: Clues to olivine–ringwoodite phase transition mechanisms in shocked meteorites and subducting slabs Ming Chen*†, Ahmed El Goresy‡, and Philippe Gillet§ *Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; ‡Max-Planck-Institut fu¨r Chemie, 55128 Mainz, Germany; and §Laboratoire des Sciences de la Terre, Centre National de la Recherche Scientifique, Unite´Mixte de Recherche 5570, Ecole Normale Supe´rieure de Lyon, 46 Alle´e d’Italie, 69364 Lyon Cedex 7, France Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved September 15, 2004 (received for review July 12, 2004) The first natural occurrence of ringwoodite lamellae was found in the chondritic portions consist of olivine, pyroxene, plagioclase the olivine grains inside and in areas adjacent to the shock veins of glass, troilite, kamacite, taenite, and accessory minerals chro- a chondritic meteorite, and these lamellae show distinct growth mite, apatite, and whitlockite. In this article, we report the mechanism. Inside the veins where pressure and temperature were discovery of natural occurrence of ringwoodite lamellae in higher than elsewhere, ringwoodite lamellae formed parallel to olivine in this meteorite. the {101} planes of olivine, whereas outside they lie parallel to the (100) plane of olivine. The lamellae replaced the host olivine from Methods a few percent to complete. Formation of these lamellae relates to Polished thin sections (35 ␮m in thickness) of Sixiangkou a diffusion-controlled growth of ringwoodite along shear-induced meteorite samples with shock-melt veins were prepared. The planar defects in olivine. The planar defects and ringwoodite mineral assemblages and textures were characterized with an lamellae parallel to the {101} planes of olivine should have been optical microscope and a Hitachi (Tokyo) S-3500N scanning produced in higher shear stress and temperature region than that electron microscope in back-scattered electron (BSE) mode. parallel to the (100) plane of olivine. This study suggests that the Compositions of minerals were quantitatively determined by a time duration of high pressure and temperature for the growth of JXA-8900RL electron microprobe (JEOL) at 15-kV accelerat- ringwoodite lamellae might have lasted at least for several sec- ing voltage and 10-nA sample current at Mainz University. onds, and that an intracrystalline transformation mechanism of Raman spectra were recorded with a Renishaw (Gloucester- ringwoodite in olivine could favorably operate in the subducting shire, U.K.) RM-2000 instrument at the Guangzhou Institute of lithospheric slabs in the deep Earth. Geochemistry. A microscope was used to focus the excitation beam (Arϩ laser, 514-nm line) to a 2-␮m spot and to collect the atural ringwoodite had been found in many shocked chon- Raman signal. Accumulations of Raman spectra lasted from 120 Ndritic meteorites, in which the ringwoodite occurs as fine- to 150 s. grained polycrystalline aggregates formed through a phase transition of olivine during exogenous dynamic events on the Results parent asteroids (1–4). It was also suggested that the olivine– We observed that some individual olivine crystals inside and ringwoodite transformation could take place in cold subducting outside the shock veins contain lamellar texture. These lamellae slabs at the deep transition zone, where pressure oversteps the were studied by using an optical microscope in reflected light and ringwoodite stability field (5–9). An experiment-produced tex- by scanning electron microscopy in BSE mode, in which the ture characteristic of this phase transition in the slabs is the lamellae display brighter than the surrounding olivine matrix. GEOLOGY formation of ringwoodite lamellae within individual olivine However, the textures of lamellae in olivine inside and within a crystals (6, 7, 10–12). A number of laboratory experiments on region Ͻ50 ␮m bordering the shock veins are distinct from those ␮ compositions like Mg2GeO4,Fe2SiO4, and (Mg,Fe)2SiO4 showed in the chondritic portion but beyond the 50- m region and at that the spinel-structured lamellae lie parallel to the (100) planes several hundred ␮m distance from the veins. The former usually of olivine (6, 10–18). These ringwoodite lamellae form platelets contains two sets of lamellae, whereas the latter contains only from several unit cells to 100 nm in thickness (6, 10–12, 14–18). one set. Fig. 1 depicts a porphyroclastic grain of olivine (0.6 ϫ The formation of ringwoodite lamellae in olivine was attributed 0.9 mm in dimension) inside the shock vein, in which two sets of either to a martensitic transformation mechanism (6, 13, 15–19) lamellae can be recognized. Individual olivine grains were or an intracrystalline nucleation and growth mechanism by replaced by lamellae from a few percent to complete. The rim of which coherent nucleation of ringwoodite occurs on the stacking the porphyroclastic olivine Ϸ200 ␮m in width contains higher faults of olivine (10–12). However, no natural occurrence of density of optically visible lamellae than the interior (Fig. 1 c and ringwoodite lamellae had been found so far either in the shocked d). Widths of lamellae at the rim are also thicker (0.3–2 ␮m, 1.5 meteorites or exhumed subducted slabs. ␮m average) than that in the interior (0.05–0.5 ␮m, 0.2 ␮m Sixiangkou meteorite is a heavily shock-metamorphosed L6- average). Fig. 2 shows a single crystal of olivine in the chondritic chondrite with a number of shock-produced veins up to 10 mm portion at a distance Ϸ150 ␮m away from the shock vein, in in thickness. The shock veins contain abundant high-pressure which only one set of lamellae can be observed. Olivine grains minerals including ringwoodite, majorite, garnet, and magnesio- with one set of lamellae usually distribute in the chondritic area wu¨stite, for which the shock-produced pressure and temperature close to the shock veins in a zone with a width Ͻ300 ␮m. of Ϸ20 GPa and 2,000°C were inferred (4). Polycrystalline Complete replacement of lamellae in the host olivine was not aggregates of ringwoodite and majorite up to 100 ␮m in size and porphyroclastic grains of olivine and pyroxene up to 1.5 mm in size distribute throughout the shock veins and are enclosed in the This paper was submitted directly (Track II) to the PNAS office. fine-grained matrix consisting of magnesiowu¨stite, majorite gar- Abbreviation: BSE, back-scattered electron. net, metal, and troilite solidified from a shock-induced chon- †To whom correspondence should be addressed. E-mail: [email protected]. dritic melt at high pressures (Fig. 1a). Outside the shock veins, © 2004 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0405048101 PNAS ͉ October 19, 2004 ͉ vol. 101 ͉ no. 42 ͉ 15033–15037 Downloaded by guest on September 27, 2021 Fig. 1. The porphyroclastic grains of olivine inside the shock vein showing two sets of ringwoodite lamellae. (a) The BSE image shows that a shock vein intersecting the chondritic portions of meteorite contains porphyroclastic grains of silicates (olivine, pyroxene, ringwoodite, and majorite) and the fine-grained matrix consisting mainly of magnesiowu¨stite, garnet, metal, and troilite. (b) Under plane polarized light, the optical picture taken at the central part of a porphyroclastic grain of olivine in the shock vein from a displays three sets of planar textures parallel to the (100), (101), and (101៮) planes. (Inset) The optical biaxial interference of a negative crystal shows that the crystal lies on a position of oblique-crossing optical axes; from it the crystallographic orientation of a and c axes can be indexed. (c) BSE image from the frame area of b shows that the planar textures parallel to (101) and (101៮) consist of ringwoodite lamellae, whereas those parallel to (100) are open fractures. Note that the interfaces of ringwoodite lamellae are distinct but locally become broader, and that the lamellae are cut off by the (100) fractures. (d) The BES image shows two sets of high-density ringwoodite lamellae at the rim of a porphyroclastic olivine from a and Ͼ95% by volume of olivine being transformed into ringwoodite lamellae. The light gray slices represent ringwoodite lamellae, and the dark areas between the lamellae are olivine. Note that these lamellae are relatively thicker than those in the interior of porphyroclastic olivine shown in c and locally become broad. observed in this type of olivine grain, and these grains contain We identify the spectrum of lamella as consisting mainly of ring- Ͻ10% by volume of lamellae. The widths of lamellae are up to woodite with traces of wadsleyite, in which the strong bands at 798 2 ␮m, mostly 0.5 ␮m. Both kinds of lamellae occur as straight and 844 cmϪ1 correspond to ringwoodite and the bands at 712 and thin platelets from a few to several hundred ␮m in length. The 918 cmϪ1 correspond to wadsleyite. The other bands at 600, 822, interfaces of lamellae with the host olivine are distinct and 852, and 955 cmϪ1 appearing in the spectrum of lamella emerge become locally broaden. from olivine (20, 21). The spectrum of olivine neighboring the The chemical compositions of the lamellae lie within the com- positional range of the bulk olivine in the chondritic portion. However, the FeO content of lamellae in an individual lamellae- bearing olivine crystal is slightly but systematically higher (22.51 Table 1. Chemical compositions of olivine and ringwoodite wt %) than that of olivine grain matrix (21.86 wt %) (Table 1). We lamellae (in wt %) identified the nature of the phase in the lamellae by Raman Bulk Olivine spectroscopy. Fig. 3 displays the Raman spectra obtained from a Oxides olivine Lamellae matrix lamella in porphyroclastic grain of lamellae-bearing olivine inside SiO 38.04 38.08 38.11 the shock veins.
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