We have collected new C data from high resolution stepped combustion analyses of 5 ureilites to better constrain their relationships with O and mineralogical/petrographic groupings. This may help the future development of petrogenetic models

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 outputsNew data on carbon isotopic compositions of someureilitesConference or Workshop ItemHow to cite:Smith„ C. L.; Franchi, Ian; Wright, Ian; Grady, Monica and Pillinger, Colin (2001). New data on carbonisotopic compositions of some ureilites. In: 32nd Lunar and Planetary Science Conference, 12-16 Mar 2001, 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/lpsc2001/pdf/1878.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.ukNEW DATA ON CARBON ISOTOPIC COMPOSITIONS OF SOME UREILITES. C. L. Smith1,2, I. A. Fran-chi1, I. P. Wright1,  M. M. Grady2 and C. T. Pillinger1.  1Planetary and Space Sciences Research Institute, The OpenUniversity, Walton Hall, Milton Keynes, Bucks, MK7 6AA, UK, 2Department of Mineralogy, The Natural HistoryMuseum, Cromwell Road, London, SW7 5BD, UK. (C.L.Smith@open.ac.uk)Introduction:  The ureilites are an unusual class ofcarbon-rich, ultramafic, achondritic meteorites, whichdisplay both primitive and evolved characteristics.  Forexample, their noble gases (which are carried largely inthe diamond phase) display a planetary or fractionatedsignature, and their O isotopes retain a heterogeneityimparted from the solar nebula [1,2].  Paradoxically,though, the mineralogical and textural characteristicsdisplayed by the ureilites are seemingly indicative ofextensive igneous processing on the ureilite parentbody (UPB), a fact which is supported by chemicalevidence such as depleted lithophile element abun-dance [3].It is clearly apparent that models aiming to resolvethe question of ureilite petrogenesis must satisfy theseemingly inconsistent chemical, mineralogical andisotopic evidence.  We have previously suggested thatstudying carbon, and the role it plays in the formationof the ureilites, may be a step forward in the under-standing of these perplexing meteorites [4].High precision O isotope analyses by Franchi et al.[5,6] indicated that the group 1 ureilites [7] may beresolved into four distinct sub-groups (A, B, C, D) onthe basis of ∆17O values. The consistency of ∆17Owithin each sub-group implies homogenisation, occur-ring either in discrete local regions on a single ureiliteparent body, or perhaps, on four separate ureilite parentbodies. A pathfinder study [6] utilising bulk carbonanalyses (samples pre-combusted at 400°C for twohours then combusted at 1000°C overnight) indicated aclear co-variation of δ13C with ∆17O. The present studyexpands on this work by undertaking a more detailedanalysis of carbon within samples from sub-groups Band sub-group D (∆17O = -0.633 ± 0.025and –0.989 ±0.025 respectively).Samples and Method: Five ureilites from sub-groups B and D have been analysed for their carbonstable isotopic compositions and overall carbon distri-bution using a combination of mass spectrometry andstepped combustion.  The five samples are; RKPA80239, PCA 82506, Sahara 99201 (provisional name),META 78008 and LEW 85328.  High resolutionstepped combustion experiments are a powerful toolwhen investigating carbon distributions and isotopiccompositions. These high resolution experiments allowdifferent species of carbon to be determined e.g. or-ganic contaminant, graphite, carbide, according tocharacteristic combustion temperatures, from a wholerock sample.  Although low temperature organic con-taminants can be removed by pre-combusting samplesat 400°C prior to a bulk analysis, the resolution of dis-tinct carbon components and isotopic compositions  isimpossible.  Importantly, carbonates are not removedby pre-combustion, which can skew both carbon yieldsand isotopic compositions, if carbonate is present in thesample. The high resolution stepped combustion analy-ses on samples from sub-groups B and D will betterconstrain carbon yields and isotopic compositions and,allow both inter- and intra- group relationships to bebetter determined.  Detailed mineralogical and pet-rographic examination of these samples is also beingcarried out which, in combination with our carbon in-vestigation, may shed light on the formation conditionsand processes occurring on the ureilite parentbody(ies).Chips were taken from internal portions of the sam-ples (to reduce the effects of contamination and weath-ering and also to exclude fusion crust) and werecrushed in an agate pestle and mortar.  Small fractionsof the powdered sample were then loaded into Pt buck-ets and high resolution stepped combustion experi-ments were carried out.  The stepped combustion ex-periments were carried out on MS86, a fully automated,high-sensitivity, static-vacuum mass spectrometer atthe Open University [8].  Experiments were carried outfrom 300°C to 1400°C with the 500°C to 900°C tem-perature range studied at high resolution (25°C steps).Results and Discussion: The results of the carbonanalyses are shown below in table 1. In all the ureilitesstudied there is a common phase liberated at tempera-tures below ~500°C with a weighted average δ13C(PDB)of -20 to -30‰, which is indicative of terrestrial or-ganic carbon [9].  Considering the presence of this lowtemperature organic contaminant the values given intable 1 below for δ13C(PDB) and wt % C are weightedaverages for temperature steps between 500°C and1300°C.Sample Sample wt.(mg)Wt % C δ13CRKPA 80239 0.271  6.9 -9.2PCA 82506 0.396 5.9 -8Sahara 99201 0.215 3.2 -2.7META 78008 0.194 1.5 -1.3LEW 85328 0.336 3.7 -2.3Table 1. Summary of average δ13C and wt% C.Lunar and Planetary Science XXXII (2001) 1878.pdfNEW UREILITE CARBON DATA:  C. L. Smith et al.Figure 1 below shows the δ13C plotted against ∆17Ofor the samples.  Other data points on the graph arefrom the pathfinder study [6].Figure 1. Co-variation of δ13C with ∆17O for group 1ureilites in sub-groups B and D.  Dashed lines = 1σ onmean.  After [6].Sub-group D: (RKPA 80239, PCA 82506, Sahara99201) These sample show the largest range in δ13C,-9.2 ‰ to –2.2 ‰, with samples falling into both the“heavy” and “light” groups according to isotopic com-position as described by Grady et al.[9].  Interestingly,samples with the lightest carbon isotopic compositioncontain the most carbon. However, two of the samples(unlabelled) are bulk analyses from the pathfinderstudy and as such the presence of any carbonate withinthem may result in unrepresentative carbon yield andisotopic compositional data.Investigation of the stepped combustion profiles forgraphite release in the samples reveals that, in additionto the relationship seen between carbon yield and iso-topic composition there is also a bi-variate relationshipbetween graphite combustion temperature and isotopiccomposition.  Figure 3 shows the stepped combustionprofiles for the three samples analysed.Figure 3. Stepped combustion profile for three sub-group D ureilites.Sahara 99201, which is the isotopically heaviest of thethree samples, also has the lowest peak combustiontemperature (650°C) and the narrowest range in com-bustion temperatures (~550-725°C). Conversely RKPA80239, which has the lightest isotopic composition ofthe samples, shows the highest peak combustion tem-perature (700°C) and the broadest range of combustiontemperatures (~575-825°C).  It is believed that thistemperature range difference is a real feature and notdue to experimental artefact or sampling effects.  Thecrystallinity of graphite influences the combustiontemperature, with greater crystallinity resulting inhigher combustion temperatures.  It may be the casethen, that graphite within Sahara 99201 is less crystal-line than that found in RKPA 80239.  All the sampleshave low shock grades [10] which would suggest thatthe elevated combustion temperatures are unlikely tobe due to the presence of diamond.Sub-group B: (META 78008, LEW 85328) Sam-ples from this sub-group show a narrower range inδ13C (-4.6 ‰ to –1.3 ‰) compared with sub-group D.However, unlike sub-group D, sub-group B shows norelationship between isotopic composition and carbonyield.  Indeed, the carbon yields for this sub-grouphave a narrower range (1.5 to 1.6 wt%).  It should benoted, however, that most of the sub-group B carbondata is from the pathfinder study [6] and as has beenmentioned previously caution should be exercisedwhen interpreting this data.It is apparent from this work that, in addition to Oisotopic composition, group 1 ureilites display markeddifferences in carbon yields and isotopes. The differ-ences in carbon between the two sub-groups wouldseem to confirm the suggestion, based on ∆17O values,that the group 1 ureilites, whilst displaying similarmineral chemistries, may have originated from discreteregions on a single body or perhaps separate parentbodies entirely.  It does not seem unlikely then, that inaddition to being heterogeneous in terms of O, that thedifferent regions on the UPB or individual parent bod-ies may also be heterogeneous in terms of carbon dis-tribution and/or isotopic composition.Continued work on ureilite carbon, in conjunctionwith detailed mineralogical and petrographic investi-gation, will help to unravel the petrogenetic history ofthese enigmatic meteorites.References: [1] Clayton, R. N. & Mayeda, T. K.(1988) GCA, 52, 1313-1318. [2] Goodrich, C. A.(1992) Meteoritics, 27, 327-352. [3] Taylor, G. J.(1993) Meteoritics, 28, 34-52. [4] Smith C. L. et al.(1999) LPS,  XXX, 1715. [5] Franchi, I. A. et al.(1997) MAPS, 32, A44. [6] Franchi, I. A. et al. (1997)LPS, XXIX, 1685. [7] Berkley, J. L. et al. (1980)GCA, 44, 1579-1597. [8] Prosser, S. J. et al. (1990)Chem. Geol. 83, 71-88. [9] Grady, M. M. et al. (1985)GCA, 49, 903-917. [10] Berkley, J. L. (1986) Mete-oritics, 21, 169-189.-1.1-1-0.9-0.8-0.7-0.6-0.5-10 -8 -6 -4 -2 0∆ ∆∆∆17 Oδ13C ‰ (rel. PDB)LEW85328META78008Sub-group        BSub-group       DSahara 99201PCA82506RKPA802390100200300400500600700800500 550 600 650 700 750 800 850RKPA 80239 PCA 82506Sahara 99201 C (ppm/°C)T (°C)Lunar and Planetary Science XXXII (2001) 1878.pdf

New data on carbon isotopic compositions of some ureilites

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New data on carbon isotopic compositions of some ureilites

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