Detailed stepped combustion of HF-HCl residues from carbonaceous and enstatite chondrites suggest Q-noble gas carrier is different from bulk carbon in the meteorites

Pillinger, Colin

Sephton, Mark

Verchovsky, Sasha

Wright, Ian

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsSeparation of Q-Noble gas carrier from bulk carbon inenstatite chondrites by stepped combustionConference or Workshop ItemHow to cite:Verchovsky, Sasha; Sephton, Mark; Wright, Ian and Pillinger, Colin (2001). Separation of Q-Noble gas carrierfrom bulk carbon in enstatite chondrites by stepped combustion. 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/1706.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.ukSEPARATION OF Q-NOBLE GAS CARRIER FROM BULK CARBON IN ENSTATITECHONDRITES BY STEPPED COMBUSTION. A. B. Verchovsky, M. A. Sephton, I. P. Wright and C. T.Pillinger. Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, UK.The nature of the planetary noble gas (PNG)carrier or Q-phase [1] in meteorites is still a subjectof discussion. Failure to identify and isolate thiscarrier since the first successful concentration of itby a factor of ~200 in an Allende HF-HCl residue[1] and establishing it to be carbonaceous in nature[2], led eventually to the so-called “labyrinth”theory [3], according to which PNG initially foundtheir way through a complex network of microporeson the surface of the carbon grains duringadsorption. Following adsorption, the labyrinthchannels could be plugged with organic molecules,preventing the release of PNG at elevatedtemperatures, or/and exchange with gases fromother reservoirs such as the terrestrial atmosphere.In support of this theory a number of rathersophisticated experimental investigations on thesorption of noble gases on carbon grain surfaceshave been undertaken [3-5], which have proved thatthe labyrinth mechanism can in principle be realisedin natural conditions. However whether or not this isthe case for PNG in meteorites remains uncertain.An important observation that fits the theory is thevery effective removal of PNG from HF-HClresidues by treatment with oxidizing acids duringwhich only a few percent of the total carbon isdestroyed. Oxidation with pure oxygen should workin a similar way, although there have been nosystematic investigations of the release of PNG andcarbon from HF-HCl residues during steppedcombustion.In the present study we decided to fill this gapby analysing a number of HF-HCl residuesseparated from meteorites of different types andmetamorphic history for noble gases, nitrogen andcarbon using detailed stepped combustion. Weanalysed three carbonaceous chondrites (Orgueil(CI), Murchison (CM) and Renazzo (CR)), threeenstatite chondrites (LEW87223 (E3), Indurch(EH4) and Yilmia (EL6)) and one ordinarychondrite Dhajala (H3.8).       Figure 1 shows a typical example of carbon andnoble gases release from carbonaceous chondrites.As can be seen, carbon and noble gases are releasedin the same temperature range, so that the completerelease of noble gases occurs only after most of thecarbon has been oxidised. More precisely, when80% of C is combusted only about 60% of planetaryAr and Xe are released. This release pattern iscompatible with PNG being a volume, rather thansurface-correlated component. A similar relationshipbetween C and PNG is also observed in mostureilites  [6], in which PNG are volume-correlated,as they located mostly in shock-produced diamondsand therefore cannot be removed by treatment withHNO3. Thus, oxidation with pure oxygen in avacuum affects the release of PNG in carbonaceouschondrites in a way significantly different from whatoccurs during the treatment of the HF-HCl residueswith nitric acid, and suggests that PNG are locatednot on the surface of any carbon grains but rather ina separate carrier (Q-phase). The specific chemicalproperties of this carrier remain, however, poorlyunderstood.C 36Ar 132Xe0510152025200 300 400 500 600 700 800Temperature oCGas release, %Figure 1.  Ar, Xe and carbon (in the form of CO2) release fromHF-HCl residue from Murchison at stepped combustion. Isotopiccompositions of the Ar and Xe within experimental errors (1-3%)correspond well to the planetary values.        A quite different and unexpected relationshipbetween the release of PNG on one hand andcarbon and nitrogen on the other is observed forenstatite chondrites (Fig. 2). In these meteorites, therelease of most of the PNG and carbon aresignificantly separated from each other with thedegree of separation depending on the metamorphicclass of the meteorites. In all of the samples, themaximum PNG release occurs at the sametemperature (about 500oC) while the release ofcarbon systematically shifts to higher temperatures(Fig. 2) with increasing thermal metamorphismfrom E3 (LEW87223) to EL6 (Yilmia). Comparedto the carbonaceous chondrites discussed above,the peak of PNG release in enstatite chondritesseems to be a bit narrower, though the releasemaximum is observed at nearly the sametemperature in both cases (Figs. 1 and 2).Lunar and Planetary Science XXXII (2001) 1706.pdfQ-noble gases in enstatite chondrites…. A.B. Verchovsky et al.      The simplest interpretation of the results is thatthere is a minor and presumably carbonaceousPNG carrier (Q-phase), which is present in all ofthe meteorites analysed in this study. It iseffectively dissolved in oxidising acids (e. g.HNO3) and oxidised with pure oxygen. Duringstepped01020304050C 36Ar132Xe200 300 400 500 600 700 800 900Gasrelaese, %Temperature oCFigure 2. Release of Ar, Xe and C from HF-HCL residue fromYilmia enstatite chondrite at stepped combustion.combustion experiments the majority ofmacromolecular carbon in carbonaceous chondrites(and perhaps in other primitive chondrites as well)overwhelms the PNG carrier as their combustiontemperatures are nearly the same. In contrasttransformation of the bulk of the macromolecularmaterial into graphite during thermalmetamorphism under reducing conditions in theenstatite chondrite parent bodies increases itscombustion temperature and separates it from thethermally-resistant PNG carrier allowing the twocomponents to be subsequently resolved duringstepped combustion. The low limit of carbon (ifany) associated with PNG can be determined fromthe results for Yilmia and amounts to <5% of thetotal carbon. The minimum 36Ar concentration inthe carrier (if it is carbonaceous) is 0.0015 cm3/gC.This is the highest concentration of Ar observed sofar in any Solar System materials.      Planetary Xe and Ar are released from theirhost phase during stepped combustion in a similarway, though in detail there are some differences. InYilmia, for example (Fig. 2), Xe is released slightlybefore Ar. This suggests a different distribution ofXe and Ar in the grains of their carrier, whichcould be a result of ion implantation, especially ifthe implantation range is comparable with the grainsizes. Similar differences for noble gasdistributions are observed in presolar diamonds forwhich ion implantation seems to be the most likelytrapping mechanism [7]. The extremely high 36Ar/Cratio in Yilmia also suggests ion implantation.       In Dhajala, PNG are also separated from mostof carbon. Our results are similar to those obtainedearlier [8].  This meteorite has a very low carboncontent (0.015% [8]) which seems to be the resultof a strong thermal metamorphism in an oxidisingenvironment. However, in contrast to thecarbonaceous chondrites, the 36Ar/C ratio inDhajala is an order of magnitude higher (but still afactor of 1.5 lower than in Yilmia), indicating thatthe PNG-carrier can survive oxidative degradationmuch better than the bulk organic matter.       Our results do not allow us to identifyconclusively the PNG carrier. We have, however,established that its combustion temperature isalmost the same in all types of chondrites andureilites. If it is carbonaceous, it has propertiesdistinct from those for the bulk carbon in themeteorites: it is not graphitised during strongthermal metamorphism under reducing conditions;it is susceptible to oxidation, but under suchconditions, survives better than the bulk meteoriticcarbon.      There have been some speculations about apossible candidate for the PNG carrier: fullerenes[9] and nanotubes [10] - both forms have cage-likestructures, which can trap noble gases. The former,however, seems not to fit the observations for thePNG concentrations [11]. The latter appears to be amore attractive hypothesis though there is no directevidence of the presence of nanotubes in meteoritesso far. Another possible form of carbon whichmight be the PNG-carrier is diamondoid. Similar tofullerenes and nanotubes these hydrocarbonorganic molecules have a cage-like structure andare highly resistant to thermal metamorphism andgraphitisation in reducing environments.       In conclusion, though we have notunequivocally identified the PNG carrier inmeteorites, we have established key physical andchemical properties that help constrain its natureand that will aid in its isolation and identification inthe future.References.  [1] R. S. Lewis et al. (1975),Science 190, 1251; [2] U. Ott et al. (1984), GCA 48,267; [3] M. J. Zadnik et al. (1985), GCA 49, 1049;{4} J. F. Wacker et al. (1985), GCA 49, 1035; [5] J.F. Wacker (1989), GCA 53, 1421; [6] T. Yamamotoet al. (1998), MAPS 33, 857; [7] A. B. Verchovskyet al. (1998), Science 281, 1165;  [8] N. Schelhaas etal. (1990), GCA 54, 2869; [9] D. Heymann (1986),J. Geophys. Res. 91, E135; [10] D. Heymann(1998), LPSC XXIX, abstr. 1098; [11] L. Becker etal. (2000), Proc. Nat. Acad. Sci. USA 97, 2979.Lunar and Planetary Science XXXII (2001) 1706.pdf

Separation of Q-Noble gas carrier from bulk carbon in enstatite chondrites by stepped combustion

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Separation of Q-Noble gas carrier from bulk carbon in enstatite chondrites by stepped combustion

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