We present results obtained by Raman spectroscopy of various organic residues of primitive chondrites in order to better characterize the microstructural state of the organic matter. These results will be correlated with the petographic classification of the chondrites

Alexander, C. M. O'D.

Busemann, H.

Fries, M.

Nittler, L.

Steele, A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsScanning micro-raman spectroscopy on carbon-richresidues of primitive chondrites: A tool for chondriteclassification and stardust analysisConference or Workshop ItemHow to cite:Busemann, H.; Alexander, C. M. O’D.; Fries, M.; Nittler, L. and Steele, A. (2005). Scanning micro-ramanspectroscopy on carbon-rich residues of primitive chondrites: A tool for chondrite classification and stardust analysis.In: 36th Lunar and Planetary Science Conference, 14-18 Mar 2005, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1975.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSCANNING MICRO-RAMAN SPECTROSCOPY ON CARBON-RICH RESIDUES OF PRIMITIVE CHONDRITES: A TOOL FOR CHONDRITE CLASSIFICATION AND STARDUST ANALYSIS   H. Buse-mann1, C. M O’D Alexander1, M. Fries2, L. Nittler1 and A. Steele2, 1Department of Terrestrial Magnetism, 2Geophysical Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC 20015, USA. (busemann @dtm.ciw.edu).   Introduction:  Organic matter in primitive meteor-ites may have mostly been formed in the interstellar medium [e.g. 1-3]. After formation of the solar system these molecules have been incorporated into the parent bodies of the various primitive meteorites [2]. Subse-quent aqueous alteration and thermal metamorphism have altered the - initially similar - set of compounds including their carbonaceous microstructures.  Fig. 1a. Raman spectrum (Allende1, image 1) with D, G and second-order Raman bands.  Fig. 1b. Raman spectrum (Murchison1, image 6) with increased background due to photoluminescence.  Raman spectroscopy is a sensitive tool to analyze the microstructure and graphitization of carbonaceous material [e.g. 4-6]. This will be important for the char-acterization of returned STARDUST samples. Single crystal graphite shows a first-order Raman shift rela-tive to the exiting wavelength (“G” band) at ωG = 1582 cm-1 (Fig. 1). Disordered finite-sized microcrystallites exhibit a “D” band at ωD ~1360 cm-1 and additional peaks, e.g. close to the G band at ~1600 cm-1. Depend-ing on the degree of crystallinity, various parameters such as the peak height ratio ID/IG or the area ratio AD/(AD+AG), the relative positions of the bands ωD and ωG and the half widths ΓD and ΓG may be useful for characterizing the analyzed material [6]. Quirico et al. [7] used Raman spectroscopy to relate the petro-graphic classification of unequilibrated ordinary chon-drites with the structural state of the carbonaceous matter, represented by the parameter pairs ΓD-ωD and ID/IG-AD/(AD+AG). We discuss here Raman spectroscopy of high pu-rity organic residues. Compared to the examination of mm-sized matrix grains [7], this may have the advan-tage of being more representative. A residue samples the organic matter from large amounts of meteoritic rock (~g). Almost pure carbonaceous matter may re-duce effects of photoluminescence and improves the Raman photon yield. We describe initial tests in order to establish a protocol that will be used to assess rela-tive metamorphic grades of more than 35 well charac-terized residues from CI, CV, CO, CM, CR, LL, H and E chondrites. These results will be presented at the meeting.   Fig. 2. This Allende image (7x7 µm) consists of 20x20 spectra. Regions with large D-bands (a) and G-bands (b). Scanned particles before (c) and after (d, not re-focused) analysis, as seen through eye view camera. The yellow and green crosses aid to estimate the parti-cle size, the red circle indicates the image center.   Experiment:  We used a new scanning Laser-induced Confocal Raman microscope WiTec α-SNOM and analyzed aliquots (~mg) of residues residing on microscope glass slides (Table 1). The following pa-rameters have been applied: Laser excitation wave-length 532 nm (frequency-doubled YAG), output laser Lunar and Planetary Science XXXVI (2005) 1975.pdfpower <22 mW, mostly <8 mW at a spot & average pixel size of ~360 nm (using a 100x SWD objective), acquisition time per spectrum 1-5 s, spectral resolution 4.56 cm-1, sample temperature 20ºC (in air). The D and G bands were fitted with Lorentzian profiles and the background with a second-order polynomial. We dis-carded spectra that show large uncertainties (>25% for I and >50% for Γ), usually those with large photolu-minescence effects and those taken on sample-poor areas. We have analyzed aggregates up to 40x40 µm in size, but also regions with particles <<1 µm.  Table 1. The analyzed samples and peak height ratios. sample agent # spectra (# images) ID/IG (avrg.±s.d.) Allende1 HF/HCl 6319 (9) 0.79±0.20 Allende2 CsF 8507 (8) 0.84±0.04 Murchison1 CsF 8656 (8) 0.36±0.08 Murchison2 (bulk) 384 (1) 0.285±0.013  Results: Typical Raman spectra with D, G and sec-ond-order bands are given in Fig 1. Fig. 1b shows the effect of photoluminescence. The background subtrac-tion can mostly account for these effects. The two Al-lende residues obtained by demineralization with HF/HCl [8] and CsF [9] do not significantly vary in the averaged D and G band parameters (Table 2). The HF/HCl residue shows merely a larger variability in the data. Hence, Cs that potentially remained from the demineralization does not lead to a significant increase of the background. The most important ΓD values are remarkably similar for Murchison bulk and the resi-due, indicating that the residues are representative for the bulk meteorites (Table 2). Similarly, the ΓD values for the Allende residues agree with ~65±11 cm-1 ob-tained for the bulk [7]. The most striking observation is the difference of ΓD and ΓG. Murchison’s values are much higher than those for Allende (Table 2). This may reflect the his-tory of the CM2 chondrites in being essentially un-heated. On the other hand, the low ID/IG ratio for Mur-chison (Table 1) implies a high metamorphic grade [6]. The metamorphic sequence found in [7] shows how-ever that the ID/IG ratio alone can not be used to non-ambiguously parameterize the metamorphic history. The D and G peak positions appear surprisingly uni-form throughout all experiments (Table 2).  The Raman microprobe technique has often been referred to as “non-destructive”. Nevertheless, al-though the laser power was set to the lowest practical values, often some carbonaceous material was ablated and small grains and fragile material was even lost. In general, however, modest variations in the laser power do not yield varying results (Fig. 3) indicating that the measurement itself does not falsify the results. This is supported by the agreement between the results ob-tained for the carbon in bulk samples and residues. The difference observable in Fig. 2c and d (large particle in the lower right) is due to a movement of the particle.  Table 2. Average peak centers ω and widths Γ for the D and G bands (in cm-1, s.d., samples as in Table 1). sample ωD ΓD ωG ΓG All1 1342±6 87±12 1588±8 67±6 All2 1343±5 76±8 1593±4 66±4 Mur1 1344±8 308±44 1588±5 109±12 Mur2 1354±6 307±28 1596±3 88±9  1300 1310 1320 1330 1340 1350 1360 137100200300400MURCHISONCsF residue image 1 "5.10" image 2 "5.15" image 3 "5.20" image 4 "5.08" image 5 "5.08" image 6 "5.10" image 7 "5.15" image 8 "5.20"D-band FWHM (cm-1)D-band (cm-1) Fig. 3. The D-band as measured on several areas of the Murchison residue. A slight variation of the laser power (see legend, arbitrary units) does not alter the results.  Acknowledgement: This work was supported by the NASA Astrobiology Institute. Allende1 was pro-duced at the ETH Zurich, Switzerland.  References:  [1] Sephton M. E. (2002) Nat. Prod. Rep. 19, 292-311. [2] Alexander C. M. O’D. et al. (1998) Meteorit. Planet. Sci. 33, 603-622. [3] Botta O. and Bada J. L. (2002) Surv. Geophys. 23, 411-467. [4] Tuinstra F. and Koenig J. L. (1970) J. Chem. Phys. 53, 1126-1130. [5] Wang Y. et al. (1990) Chem. Mater. 2, 557-563. [6] Wopenka B. and Pasteris J. D. (1993) Am. Mineralogist  78, 533-557. [7] Quirico E. et al. (2003) Meteorit. Planet. Sci. 38, 795-811. [8] Buse-mann H. et al. (2000) Meteorit. Planet. Sci. 35, 949-973. [9] Cody, G. D. et al. (2002) GCA 66,1851-1865.  Lunar and Planetary Science XXXVI (2005) 1975.pdf

Scanning micro-raman spectroscopy on carbon-rich residues of primitive chondrites: A tool for chondrite classification and stardust analysis

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Scanning micro-raman spectroscopy on carbon-rich residues of primitive chondrites: A tool for chondrite classification and stardust analysis

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