Bérczi, Szaniszló

Gucsik, Arnold

Gyollai, I.

Nagy, Sz.

Veres, Miklós

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Evidence of Shock Metamorphism in Hungarian L-chondrites using Raman Spectroscopy. Sz. Nagy1, A.Gucsik2, Sz. Bérczi1, M. Veres3, I. Gyollai1, J. Fürj1 1Eötvös University, Faculty of Science, Institute of Physics,Dept. Material Physics, H-1117 Budapest, Pázmány Péter sétány 1/a, Hungary, 2Max Planck Institute forChemistry, Dept. of Geochemistry, Joh.- J. Becherweg 27, D-55128, Mainz, Germany, 3SZFKI-KFKI, H-1121Budapest, Konkoly-Thege M. út 29-33.Introduction: The Mócs and Mezőmadarasmeteorites felt in 1882 and 1852, respectively. TheMócs meteorite is an L5 and Mezőmadaras is anL3-type chondrites. The chemical composition ofthese samples were determined the ElectronMicroprobe Analyses [1]. The shock-metamorphicclassification of Mócs and Mezőmadaras meteoriteswere classified as S3-S5 for Mócs, and S2-S3 forMezőmadaras by Optical Microscope [2]. We havereinvestigated of these samples using Ramanspectroscopy for the identification of any shock-metamorfic effects occured in the structure changesof olivine and pyroxene grains. The detailed OManalyses show mechanical twins in pyroxenes (Fig.1), planar fractures (PF’s), and planar deformationfeatures (PDF’s) (Fig. 2). Furthermore shockmosaicism in olivine grains is also discernible (Fig.3). The Raman spectroscopical investigationrevealed β-(Mg, Fe) SiO4 olivine structure(wadsleyite) in a PDF’s enriched and highlydeformed olivine grain.Samples and Experimental ProcedureThe mineral assemblages and textures werecharacterized by a Renishaw-1000 Ramanspectrometer, the laser wavelenght was 785 nm,with focused energy of 8 mW. The maximal focuswas driven to 1µm spot in diameter. The thinsections were mounted in epoxy material, and thesample thickness is 30 µm.Fig. 1 Optical microscope (cross-polarised) imageof pyroxene with mechanical twins inMezőmadaras meteorite.Results and DiscussionMócs meteorite: In a highly deformed and PDF’senriched olivine grain was selected for the Ramaninvestigations representing two analyzing points, asfollows. The first point (Mócs I.) was placed on thePDF’s lamellae, the other points (Mócs II.) was inbetween PDF’s (Fig. 2).Fig. 2 Places of two points for ramanspectroscopical analysis in PDF’s rich olivine grainfrom Mócs meteorite. Reflected light micrographimage.The characteristic Raman peaks of Mócs I are: 630(strong-s), 706 (medium-m), 722 (very weak-vw),751 (weak-w), 852 (very strong-vs), 883 (verystrong-vs), and 987 (strong-s) cm-1. The Mócs II-spectrum contains the following peaks: 421 (s), 577(m), 642 (w), 822 (vs), 854 (vs), 920 (m), and 958(m) cm-1. We used a reference material from theTrudy’s mine olivine to the comparison with thevibrational changes of our spectra (Fig. 4). Notethat the spectrum was derived from CALTECHRaman database. The most characteristic changes inthe Mócs I-spectrum are. The olivine doublet peakposition is shifted to higher wavenumbers, due topartly disordered olivine structure. The doubletpeak’s shift was also observed in olivine(crystalline forsterite) experimentally shockedbetween 0-50 GPa [3]. The second importantchange in the Mócs I-spectrum is in the middlewavenumber region. The olivine structurevibrations are forbidden in this region. The 751 cm-1 peak is assigned to the presence of Si-O-Si bridgescharacteristic of wadsleyite [3,4] indicating thehighly deformed olivine structure [2]. The 722 cm-1peak belongs to the wadsleyite Si2O7 symmetricstretching vibrational mode [5,6]. However, the 918cm-1 peak is not identical to the wadsleyite SiO3symmetric stretching vibrational mode, because ofthe doublet peak position.Mezőmadaras meteorite: Mezőmadaras samplecontains poikilitic pyroxene condrums, which showelongated shape. Within these condrums, thepyroxenes have mechanical twins.3012.pdfMicro-Raman Spectroscopy and Luminescence Studies (2009)Fig.  3. Moderately mosaicism in olivine fromMócs sample. OM-imageWithin the most common (Mg-Fe-Ca) pyroxeneswe can distinguish more than ten Raman activevibrational modes [7]. The four spectra (Fig. 5)show different chemical composition between theCastle Rock (as a reference spectrum) and threemeteorites sample. The Mócs spectrum have thefollowing peaks: 368 (vs), 426 (s), 475 (m)?, 693(vs), 710 (vs), 1040 (vs) cm-1. The Mezőmadarasspectrum contains peaks: 372 (s), 475 (w)?, 694(vs), 716 (vs), and 1041 (vs) cm-1. Thecharacteristic peaks of Shergotty pyroxenes are:359 (vs), 426 (vs), 567 (vw), 590 (vw), 698 (vs),and 1043 (vs) cm-1. The Castle Rock pyroxeneexhibits the following peaks: 340 (vs), 402 (s), 439(s), 538 (m), 661  (vs), 682 (vs), 860 (m), 936 (w),and 1009(vs) cm-1.Fig.  4. Raman spectra of olivine in Mócs meteoritesample.The degree of the structural disordering wasdetermined by FWHM (Full Width at HalfMaximum) values. Under the 500 cm-1 range theMócs sample, has the highest rate of FWHM as 17cm-1 but the Castle Rock sample shows 11 cm-1.The Mezőmadaras and Shergotty samples arebetween two of the above mentioned samples(FWHM rate is 15 cm-1). This indicates thesignificant changes in the Si-O stretching modesand SiO4 tetrahedra rotational modes. Above 800cm-1 range, the FWHM rates are: Mócs 21 cm-1),Shergotty 20 cm-1), Mezőmadaras 18 cm-1), andCastle Rock 15 cm-1). These data represent higherstructure disordering degree in meteorites than inthe terrestrial Castle Rock sample. The higherFWHM rate in meteorites can be due to shock-metamorphic mechanism.Fig.  5. Raman spectra of pyroyenes from differentsources.On the basis of the measured FWHM, the followingstructural disordering degree of the selectedsamples can be determined as a (growing rate):Castle Rock, Mezőmadaras, Shergotty and Mócs.The result reveals that amorphisation rate is thehighest in Mócs and Shergotty pyroxene grains.ConclusionThis study demonstrates that the micro-Ramanspectroscopy can aid to understand about the studyof shock-induced phase transformation and range ofstructural disordering in minerals from variousmeteorites.AcknowledgementWe are grateful for Professors I. Kubovics forhelpful discussions, and for Raman Laboratory ofSZFKI (Budapest, Hungary) for instrumentalbackground.References[1] Kubovics et al., (1997) NIPR Symposium,Tokyo  pp. 13-14.[2] http://www.museum.hu-berlin.de/min/forsch/Schockklassifikation%20Stoeffler/Final%20Version/subdir/list.htm [3] Durben et al., (1993) AmericanMineralogist, Volume 78, pp. 1143-1148. [4] Gilletet al., (2005) Meteoritics & Planetary Science 40,pp. 1175-1184. [5] Kleppe et al., (2006) AmericanMineralogist, Volume 91,  pp. 1102-1109. [6]Kleppe et al., (2002) Physics and Chemistry ofMinerals, 29, 376-473. [7] Huang et al., (2000)American Mineralogist, Volume 85, pp. 473-479.3012.pdfMicro-Raman Spectroscopy and Luminescence Studies (2009)

Evidence of Shock Metamorphism in Hungarian L-Chondrites Using Raman Spectroscopy

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