Franchi, Ian

Grady, Monica

Pillinger, Colin

Smith, C. L.

Wright, Ian

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsUreilite Graphite: Shocking implicationsConference or Workshop ItemHow to cite:Smith, C. L.; Franchi, Ian; Wright, Ian; Grady, Monica and Pillinger, Colin (2001). Ureilite Graphite: Shockingimplications. In: 64th Annual Meteoritical Society Meeting, 10-14 Sep 2001, Vatican City, Italy.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/metsoc2001/pdf/5366.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.ukUREILITE GRAPHITE: SHOCKING IMPLICATIONS. C. L. Smith1, 2, I. A. Franchi1, I. P. Wright1, M. M.Grady2, C. T. Pillinger1. PSSRI, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK,(C.L.Smith@open.ac.uk), 2Mineralogy Department, The Natural History Museum, London, SW7 5BD, UK.Introduction: The ureilites are C-rich, ultramaficachondrites whose petrogenesis remains ambiguous.Paradoxically they display both differentiated (e.g.igneous textures) and primitive characteristics (e.g. Omass independent isotope signatures) [1,2].The carbon in the ureilites clearly plays an impor-tant role in their petrogenetic history although its ori-gin is a subject of controversy. It may be a primaryfeature retained from a carbonaceous chondrite (CC) -like precursor material or it may be a secondary featureinjected into the juvenile ureilite parent body (UPB)during a planetisimal scale impact event [3,4]. Wehave previously reported initial results from a studyinto ureilite carbon [5]. Here we report new data fromboth ureilites and terrestrial graphite samples whichindicate a clear relationship between the form ofureilite C and its isotopic composition.Method & Results: Ureilites, and terrestrialgraphite samples of known crystallinities (determinedby both XRD analysis and Raman spectrography),were analysed using a combination of high-resolutionstepped combustion and mass spectrometry [5].The terrestrial graphite samples show a clear rela-tionship of increasing temperature range over the mainrelease, from 175 to 275°C with increasing crystallin-ity, and an increase in peak release temperature from652 to 864°C.  The ureilite data show good trends ofincreasing range in release temperature and ä13Cagainst C content. These variations also parallel in-creasing shock levels in the samples. This variation ofdecreasing C content and increasing ä13C (more accu-rately range in ä13C) with release range (i.e. degree ofshock) indicates the operation of more than one con-trolling factor. No obvious correlation between peakrelease temperature and any other parameters is clearin the ureilite samples suggesting that the releasebroadening is a function of increasing heterogeneity inthe graphite’s crystallinity rather than simply increas-ing crystallinity per se.Shock may be an appropriate mechanism to in-crease the heterogeneity of graphite crystallinity assimulations show that although graphite becomesstructurally disordered during shock the propagation ofthe shock wave is complex and heterogeneity occurseven on a very small scale [6].Discussion: A number of possibilities may accountfor the relationships seen within the ureilites.It has been suggested previously that amorphous orsemi-ordered ureilite graphite may be a relic of primi-tive C material inherited from a CC-like precursormaterial [7]. This being the case low crystallinity Cenriched in 12C may be primary ureilite graphite, whichwas then mixed with variable amounts of impactor Cenriched in 13C. Mixing variations will result in in-creasing modification of C content and isotopic com-position.Alternatively, modification of the indigenous car-bon could occur during high temperature conditions onthe UPB (as a result of shock and/or internal heating).These conditions may result in preferential loss of 12C-rich volatiles (CO2) through interaction of graphitewith surrounding material e.g. during smelting reac-tions. The extent of heating will influence the degreeof reaction causing a variation of both ä13C and crys-tallinity in the resulting graphite. If the heating isshock induced then the extent of the reaction will varywith shock level.Acknowledgements: Gordon Cressey (NHM) iskindly thanked for XRD analysis.References: [1] Taylor G. J. (1993) Meteoritics,28, 34-52. [2] Mittlefehldt D. W. et al. (1998) inPlanetary Materials (Ed. Papike J. J.) Reviews in Min-eralogy v. 36, 4-73-4-95. [3] Clayton R. N. & MayedaT. K. (1988) GCA, 52, 1313-1318. [4] Warren P. H. &Kallemeyn G. W. (1989) Meteoritics, 24, 223-246. [5]Smith C. L. et al. (2001) LPSCXXXII, 1878. [6] Ya-mada K. et al. (2000) Phil. Magazine A, 80, 1811-1828. [7] Kagi H. et al. (1991) Proc. NIPR Symp. Ant-arct. Meteorites, 4, 371-383.64th Annual Meteoritical Society Meeting (2001) 5366.pdf

Ureilite Graphite: Shocking implications

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Ureilite Graphite: Shocking implications

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