Raman spectroscopy suggests that the C was emplaced in at least two separate episodes into the impactites of the Gardnos impact structure

Franchi, I. A.

French, B. M.

Gilmour, I.

Koeberl, C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsA Raman spectroscopic study of carbon phases inimpact melt rocks and breccias from the Gardnosimpact structure, NorwayConference or Workshop ItemHow to cite:Gilmour, I.; Franchi, I. A.; Koeberl, C. and French, B. M. (2002). A Raman spectroscopic study of carbonphases in impact melt rocks and breccias from the Gardnos impact structure, Norway. In: 33rd Lunar and PlanetaryScience Conference, 11-15 Mar 2002, 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/lpsc2002/pdf/1623.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.ukA RAMAN SPECTROSCOPIC STUDY OF CARBON PHASES IN IMPACT MELT ROCKS ANDBRECCIAS FROM THE GARDNOS IMPACT STRUCTURE, NORWAY. I. Gilmour1, I.A. Franchi1, C. Koe-berl2 and B.M. French3, 1 Planetary and Space Sciences Research Institute, The Open University, Milton KeynesMK7 6AA, U.K., 2 Institute for Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria,3Smithsonian Institution, Washington, DC 20560, U.S.A (I.Gilmour@open.ac.uk)Introduction: The Gardnos impact structure [1] islocated in southern Norway (60°40´N; 9°00´E), ap-proximately 125 km northwest of Oslo, Norway.  Thestructure, originally about 5 km in diameter, has beendeeply eroded.  It lacks any distinctive circular formand is now represented by outcrops of impact-produced breccias, both lithic and melt-bearing.  Itsimpact origin has been convincingly established fromthree lines of evidence [1]:  (1) the presence of melt-bearing breccias containing granitic rock fragmentswith definite Planar Deformation Features (PDFs) inquartz and feldspar; (2) the demonstration, usingchemical mixing models, that the breccia compositionscould be produced by mixing the exposed target rocks;(3) an extraterrestrial signature from the projectile inthe breccias, established using osmium-isotope analy-ses.Figure 1. Photomicrograph of a carbonaceous clast in asuevite from the Gardnos impact structure. Scale bar is100µm.A major unresolved problem about the Gardnos struc-ture is the unusually high content of carbon in its im-pact-produced rocks (Fig. 1).  Gardnos is one of onlytwo known impact structures (among approximately175) whose impactites contain significant amounts(typically 0.2-1.0 wt%) of carbon, or 5-10 times theamount present in the target rocks; Sudbury, Canada isthe other [2].  Values of δ13C for the Gardnos carbonrange from –28 to –32 ‰ [1], strongly suggesting anorigin from biogenically derived carbon. We have pre-viously suggested the carbonaceous Proterozoic BiriShale, which could have been present overlying thebasement rocks at the impact site, as a possible carbonsource [1].Experimental: Powdered samples (ca. 4g) were dem-ineralized using procedures described previously [2]with the exception that microwave assisted dissolutionwas used for the initial removal of silicates. The dem-ineralized residues were also sub-sampled and treatedwith chromic acid (6 M at 70°C) to remove amorphousand organic carbon. Aliquots of this residue were thentreated with fuming perchloric acid at 110°C to removecrystalline graphite. The sequence of residues so pro-duced were studied using (1) a transmission electronmicroscope which was also to obtain selected-areaelectron diffraction (SAED) patterns to confirm theidentity of carbon components. (2) High sensitivitystable isotope mass spectrometery and (3) RamanSpectroscopy. Here, we report the results of the Ramaninvestigation.Raman Spectroscopy: For all measurements a Jobin-Yvon Labram HR laser Raman microprobe system wasused. Excitation was with a 40 mW 514 nm Ar-ionlaser (amount of power on sample <8mW). Spectrawere obtained using a 50 X objective and a 600grids/mm grating from 600 cm-1 to 2300 cm-1 with aspectral resolution of 2 cm -1. The detection time wasvaried form 1 to 50 s, but most of the samples wereanalyzed with a detection time of 30 s. Peak areas,widths, positions and ratios were determined usingJobin-Yvon Labspec software.Results: First order spectra were analyzed from 600 to2300 cm-1 for carbonaceous residues from 5 samplestogether with spectra from a suite of crystalline graph-ite standards. Between 3 and 7 spectra were measuredfor carbonaceous material from each sample. Thespectra from the Gardnos samples all contained thefirst-order single band at around 1582 cm-1 (O-peak)that is characteristic of well-crystallized graphite. Dis-order in graphite appears as a broadening of the 1582cm-1 band together with a shift toward higher wave-numbers as a result of the development of an additionalband near 1360 cm-1 (D-peak) [3].  With the exceptionof a chemically isolated graphite sample from thesuevite NGF-96-137 carbon from all of the Gardnosimpactite samples studied show two large peaks, atabout 1582 cm-1 and 1360 cm-1. Carbonaceous materialfrom suevite NGF-96-178 also showed a shoulder de-veloped on the high wavenumber side of the 1582 cm-1peak indicating disorder in the graphite. Carbonaceousmaterial from the Biri shale gave much broader 1582Lunar and Planetary Science XXXIII (2002) 1623.pdfRAMAN SPECTROSCOPY OF CARBON FROM THE GARDNOS CRATER:  I. Gilmour et al.cm-1 peaks (shifted to high wavenumbers around 1597cm-1) together with a broad 1360 cm-1 peak.1560157015801590160016100 20 40 60 80Peak width (cm-1)Peak Position (∆cm-1 )Graphites Biri shaleGardnos Breccia Suevite NG-96-133Suevite NG-96-178 Suevite (graphite-rich residue)Figure 2. Position of the O-Raman peak (corresponding toordered carbon) vs. O-peak width for isolated C grains fromGardnos impactites and potential C-rich target rocks. Shownfor comparison are a series of crystalline graphite standards(open triangles).A range of ordering can be identified for the carbonpresent in Gardnos impactites from poorly ordered Cin the Biri shale to crystalline graphite in the isolatedresidues from suevite NG-96-137. The range in order-ing of the C is shown in Figure 2 which plots the posi-tion of the 1582 cm- 1 O-peak against its width.Graphitic material in the Gardnos Breccia and Suevitesis more ordered than in the Biri shale with some sam-ples approaching the crystallinity of well-orderedcrystalline graphite.Origin of carbon: The variations in graphite orderingin Gardnos impactites, which range from well-crystallized graphite to more poorly-ordered C, mayreflect more than one episode of C-emplacement. TheRaman spectra of graphite formed in-situ from themetamorphism of organic matter can be used to esti-mate the rock's peak metamorphic temperature [3].Graphitic material produced by the progressive meta-morphism of organic matter shows an increase incrystallinity with metamorphic grade. In contrast,graphite deposited due to nucleation and crystal growthfrom fluids generally does not show the same degree ofordering as metamorphic graphite subjected to thesame temperatures. Metamorphic temperatures atGardnos reached 350-400°C during the Caledonianorogeny, consistent with the degree of ordering ob-served in Raman spectra of carbonaceous materialfrom the Biri shale. The higher degree of ordering inmuch of the graphitic material in Gardnos suevites andBreccias suggests that this C was either derived frommore crystalline graphite incorporated into these rocksduring the impact event or that it has experienced tem-peratures higher than those due to the regional meta-morphism of the Caledonian orogeny. The more crys-talline nature of graphite in the suevites and brecciasalso apparently precludes a fluid-deposited origin dur-ing regional metamorphism. Andersen and Burke [4]measured Raman spectra for carbonaceous material inGardnos quartzite that were very similar to the spectraobtained in this study for the Biri shale, i.e., C with alower degree of ordering than the C present in thesuevites and breccias but consistent with the tempera-tures reached during the Caledonian orogeny. Thissuggests that the degree of ordering of C in Gardnosimpactites may be related to post-impact temperatureswith C in suevites having the greatest degree of order-ing, breccias less ordered, and shocked quartzites theleast ordered. Andersen and Burke [4] also concludedthat hydrocarbon fluid inclusions present in the quartz-ite were trapped at  temperatures of around 320 °C andpressures of around 2.5 kBar, trapping conditions con-sistent with conditions during the Caledonian orogenybut not with the higher temperatures and lower pres-sures associated with post-impact cooling.Conclusions: The variations observed in the orderingof C in Gardnos impactites suggest that there were atleast two episodes of C emplacement in Gardnos im-pactites. An initial impact-related incorporation andshock transformation of graphitic material from targetrocks followed by later mobilization of C.References:  [1] French B. M., Koeberl C., Gilmour I.,Shirey S. B., Dons J. A., and Naterstad J. (1997) Geochim.Cosmochim. Acta 61, 873-904. [2] Gilmour I., Russell S. S.,Arden J. W., Lee M. R., Franchi I. A., and Pillinger C. T.(1992) Science 258, 1624-1626. [3] Wopenka B. and PasterisJ. D. (1993) Am. Mineral. 78, 533-557. [4] Andersen T. andBurke E. A. J. (1996) European J. Min. 8, 927-936Lunar and Planetary Science XXXIII (2002) 1623.pdf

A Raman spectroscopic study of carbon phases in impact melt rocks and breccias from the Gardnos impact structure, Norway

http://oro.open.ac.uk/8164/1/A_Raman_Spectroscopic_Study_of_Carbon_Phases_in_Impact_Melt_Rocks_and_Breccias_from_the_Gardnos_Impact_Structure%2C_Norway.pdf

A Raman spectroscopic study of carbon phases in impact melt rocks and breccias from the Gardnos impact structure, Norway

Abstract

Similar works

Full text

Available Versions

Open Research Online (The Open University)