We have determined the abundance and isotopic composition of two separate carbonate generations within Tagish Lake. Nitrogen isotope analysis of the chondrite shows it to be organic-rich, with d15N values higher than in CI and CM chondrites

Franchi, I.A.

Grady, M.M.

Pillinger, C.T.

Verchovsky, A.V.

Wright, I.P.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsContinued investigation of the light elementgeochemistry of Tagish LakeConference or Workshop ItemHow to cite:Grady, M.M.; Franchi, I.A.; Verchovsky, A.V.; Wright, I.P. and Pillinger, C.T. (2001). Continued investigationof the light element geochemistry of Tagish Lake. 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/1733.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.ukCONTINUED INVESTIGATION OF THE LIGHT ELEMENT GEOCHEMISTRY OF TAGISH LAKE.Monica M. Grady1, I. A. Franchi2, A. B. Verchovsky2, I. P. Wright2 and C. T. Pillinger2. 1Natural History Mu-seum, Cromwell Road, London SW7 5BD (mmg@nhm.ac.uk); 2PSSRI, The Open University, Walton Hall, MiltonKeynes MK7 6AA.Introduction: Preliminary analysis of the lightelement stable isotope geochemistry of the TagishLake primitive carbonaceous chondrite showedthat it was a carbonate-rich meteorite containingabundant presolar grains [1, 2]. Tagish Lakeseemed to be more carbon-rich than CI chondrites,with over 5 wt. % carbon distributed almostequally between organic and inorganic (mostlycarbonate) components. We have continued ourinvestigation of this unusual meteorite, turningattention to (i) the abundant carbonates; (ii) thedistribution of nitrogen between organic and inor-ganic components and (iii) pre-solar grains.Isotope analyses were carried out at the OpenUniversity. The abundance and isotopic composi-tion of carbonates were obtained by sequentialdissolution of two 30 mg aliquots taken from apowdered 300 mg sample of pristine Tagish Lakematerial. The carbon and oxygen isotopic compo-sition of the carbonates were analysed by two-stage orthophosphoric acid dissolution [3, 4]. Thefirst extraction, performed at 25°C primarily re-leases calcite, whilst the second, at 75°C, attacksdolomite/siderite. After purification, the CO2 liber-ated was analysed on a PDZ Europa Geo 20-20mass spectrometer; d13C and d18O were measuredto precisions of ± 0.5‰. Nitrogen and argon re-sults were obtained simultaneously by steppedcombustion (room temperature to 1400°C in stepsof 25°C) of a single chip of Tagish Lake weighing4.40 mg. Isotopic compositions were measured onthe Finesse noble-gas type static vacuum massspectrometer to precisions of ± 1‰.Carbonates: petrographic analysis of TagishLake has shown that areas of the chondrite are richin fine-grained carbonates, both Ca- and Fe-rich[2]. Initial measurement of the carbonates bythermal decomposition indicated that they wereenriched in 13C, around 3.2 wt.% carbon as car-bonate, with d13C between +45‰ and +65‰, andFe, Mg carbonate approximately twice as abun-dant as calcite [1]. Sequential acid dissolution ofwhole rock Tagish Lake has allowed better dis-tinction to be drawn between the different carbon-ates present (see table). The overall carbonateyield is lower, by a factor of three, than that de-termined from thermal decomposition, and carbonisotopic composition higher. Reasons for the dif-ference between the two sets of results are proba-bly due to sample heterogenity and failure of thestepped combustion method to resolve adequatelythe organic and carbonate components. Acid dis-solution results show that Fe,Mg carbonates domi-nate over Ca carbonates, confirming petrographicand combustion observations. However, the differ-ence in d13C between the two generations of car-bonate, as seen from stepped combustion, is notmirrored by the dissolution experiments.Table: Carbonate analysis by acid dissolution(mean of two analyses)Yield d13C d18O(ppm) (‰) (‰)Ca (25°C) 2340 +67.4 +31.9Fe,Mg (75°C) 10500 +67.6 +35.6Total 12840 +67.6 +35.2The total isotopic composition of Tagish Lakecarbonates (Fe, Mg and Ca) is slightly heavierthan any previously reported carbonaceous chon-drites (Figure 1), and abundance is higher than inany other chondrite group [4]. Carbonates in CMsshow no apparent co-variation of d13C and d18Oand no clear relationship to those in Tagish Lake.But curiously, this new sample falls at the extremeend of a trend of increasing d13C with d18O for theCIs. Zito et al. [5] noted a similar relationshipbetween calcite, dolomite and breunnerite inOrgueil; it is possible that the results from TagishLake signify a higher abundance of calcite relativeto the Fe- and Mg-rich carbonates compared toCIs. If this is the case, it raises the possibility thatTagish lake carbonates share some common originor history to those in CI carbonaceous chondrites.Lunar and Planetary Science XXXII (2001) 1733.pdfLIGHT ELEMENT GEOCHEMISTRY OF TAGISH LAKE:  M. M. Grady et al.Figure 1: Isotopic composition of carbonates fromCI and CM chondrites [4]02040608010 20 30 40Tagish Lake CMCId13C(‰)18d O (‰)Nitrogen: Tagish Lake has a nitrogen abun-dance between CI and CM, with an isotopic com-position more 15N-enriched than CI or CM chon-drites: [N] = 1218 ppm; d15N ~ + 67‰. As shownin Figure 2, there are several nitrogen componentspresent in Tagish Lake. Most nitrogen combustsbelow around 600°C. Up to 400°C, nitrogen ex-hibits a practically monotonic increase in d15Nwith temperature. Most of this material is fromorganic matter – that released at the lowest tem-peratures (below around 300°C; d15N ~ +40 to+60‰) is from more labile components such asamino acids, whereas that combusting at slightlyhigher temperatures (around 350 - 500°C; d15N upto +70‰) is from the highly-cross linked, aromaticmacromolecular material that comprises most ofthe carbon in carbonaceous chondrites. Superim-posed on the nitrogen released from organic mate-rials is nitrogen from interstellar nanodiamonds.The presence of nanodiamonds can be inferredfrom the steep drop in d15N between 475°C and525°C, where 14N-enriched nitrogen is released asthe nanodiamonds combust. When compared withthe corresponding carbon data [1], the presence ofinterstellar graphite and silicon carbide grains isalso shown by excursions in d15N: up to ~ +90‰at 800°C (combustion of graphite) and down to0‰ at 1100°C (SiC).Comparison with nitrogen data acquired fromOrgueil and Murchison shows that Tagish Lake isslightly depleted in nitrogen at the lowest tem-peratures, and d15N of the remaining nitrogen isalso lower. This is probably a result of loss ofsome of the most water-soluble organics, including15N-enriched amino acids, during either processingon Tagish Lake’s parent body, or during the de-frosting process prior to analysis.There has been much discussion on the originof organic nitrogen in carbonaceous chondrites,whether it is nebular (e.g., ref. 6) or remnant inter-stellar material [7]. Recognition that interstellarnanodiamonds account for an approximately con-stant percentage of carbon in carbonaceous chon-drites (once parent body processing has been takeninto account) has led to the suggestion that muchof the organic material might also be presolar inorigin [7]. Since Tagish Lake seems to be rela-tively enriched in nanodiamonds (with maybe up to20% of the total carbon coming from this source),it is an ideal sample to study to pursue potentialrelationships between organic and inorganic nitro-gen-bearing components. To this end, a suite ofacid-resistant residues are being prepared, for highresolution carbon and nitrogen isotope analysis.Figure 2: Nitrogen in Tagish Lake0123450204060801000 200 400 600 800 1000 1200 1400     [N](ppm/°C)Temperature (°C)15N(‰)dReferences: [1] Grady et al. (2000) MAPS 35,A62-A63; [2] Brown P. G. et al. (2000) Science290, 320-325; [3] McCrea J. M. (1950) J. Chem.Phys. 18, 849-857; [4] Grady M. M. et al. (1988)GCA 52, 2855-2866; [5] Zito K. L. et al. (1998)MAPS 33, A171-A172; [6] Geiss J. and Bochsler,P. (1982) GCA 46, 529-548; [7] Alexander C. M.O’D. et al. (1998) MAPS 33, 603-622.Lunar and Planetary Science XXXII (2001) 1733.pdf

Continued investigation of the light element geochemistry of Tagish Lake

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Continued investigation of the light element geochemistry of Tagish Lake

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