Analysis of noble gas element variations in the P3 component of presolar diamonds indicates that a special mechanism of element fractionation is required to explain the variations

Fisenko, A. V.

Pillinger, C. T.

Semjonova, L. F.

Verchovsky, A. B.

Wright, I. P.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOn the variations of the elemental composition of theP3 component in presolar diamondsConference or Workshop ItemHow to cite:Fisenko, A. V.; Verchovsky, A. B.; Semjonova, L. F.; Wright, I. P. and Pillinger, C. T. (2004). On the variations of theelemental composition of the P3 component in presolar diamonds. In: 35th Lunar and Planetary Science Conference,15-19 Mar 2004, 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/lpsc2004/pdf/1665.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.ukON THE VARIATIONS OF THE ELEMENTAL COMPOSITION OF THE P3 COMPONENT IN PRESOLAR DIAMONDS.  A. V. Fisenko1, A. B. Verchovsky2, L. F. Semjonova1,  I. P. Wright2 and C. T. Pillin-ger2, 1Vernadsky Institute of Geochemistry and Analytical Chemistry RAS,  Moscow, Russia, anatolii@fis.home.chg.ru,, 2The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.   Introduction:  In the present study we focus mostly on the effects of parent body thermal metamorphism on variations of the elemental composition of P3 noble gases and the corre-sponding variations in release characteristics of the noble gases. In particular we analyse element variations in the low (<800oC) and high (>800oC) temperature parts of the P3 gases  (labeled below as P3-L and P3-H respectively), released during stepped pyrolysis. This aspect of P3 noble gases behavior in presolar diamonds has not been ana-lysed in detail so far. Below we use the data pub-lished in [1]. Results: Element variations in P3 gases:  The elemental composition of the P3 component in nanodiamonds from unprocessed meteorites is different from that present in samples that are known to have been thermally metamorphosed. The most primitive meteorite in terms of thermal processing is thought to be Orgueil. As can be seen from Figure 1, in all metamorphosed meteor-ites the P3 component  tends to have a depletion of Kr and an enrichment of Ar relative to Xe com-pared to the abundance of the elements in P3 of Orgueil. The same trend is observed in Figure 2 where the integral amounts of all types of gases released up to 800oC are shown: experimental points for the most metamorphosed meteorites deviate to the left from the HL-P3 mixing line. For each separate meteorite the elemental compositions of P3-L and P3-H are different (Fig. 1). This difference  seems to be dependent on the metamorphic history. For instance, for Orgueil, a thermally unprocessed but significantly aqueous altered meteorite, P3-L and P3-H have similar Kr/Xe but different Ar/Xe ratios. Similar varia-tions are observed for Semarkona and Bishunpur. The most significant differences between P3-L and P3-H (in terms of Kr/Xe ratio) are observed for the thermally processed meteorites such as Kainsaz, Leoville and Vigarano. However, varia-tions in Kr/Xe ratios for Vigarano are opposite to those observed for Kainsaz and Leoville. We speculate that it is a particular combination of mild aqueous alterations with thermal processing in Vigarano which is responsible for the differ-ence. For the more strongly metamorphosed Al-lende and Indarch, representing correspondingly oxidised and reduced groups, Kr/Xe ratios for P3-H are extremely different (Allende is similar to Vigarano, while Indarch is similar to Kainsaz and Leoville). Discussions:  An important point that follows from the observations presented above is that if Orgueil is indeed the least thermally processed solar system material (i.e. its P3 gases most really represent the true P3 composition) then the ele-mental variations between  noble gases of the component observed during parent body meta-morphism and laboratory step heating cannot be simply explained by diffusion fractionation. We suggest two possibilities to explain this paradox.  The first possibility suggests that, in spite of low temperature processing, Orgueil material has, in fact, lost most of its original P3 component in a process resulting in element fractionation (diffu-sion-like separation).  This would mean that the true P3 composition must have much higher Kr/Xe and Ar/Xe ratios. On the plot (Fig. 1) the point corresponding to this hypothetical composi-tion must lie somewhere in the upper right corner. In this case all the other bulk P3 compositions, corresponding to the thermally and aqueous al-tered meteorites, would be the result of diffusive element separation in which  lighter elements were lost preferentially to the heavier ones. The  spread of the points on the plot (Fig. 1) should then be explained by particular conditions of metamorphism in different parent bodies. As far as the variations in the compositions of  P3-L and P3-H are concerned, one could suggest that they reside in diamond populations of differ-ent origins and hence P3 compositions. Such an explanation is supported by the observation [2] that release temperature of implanted noble gases strongly depends on the total radiation dose ac-quired by diamonds (i.e. on the number of defects produced in their structure). Therefore, variations in  P3-L and P3-H compositions cannot be com-pared directly since the diamond populations, Lunar and Planetary Science XXXV (2004) 1665.pdfwhere these components are located, can also have a different response to metamorphism. The alternative possibility suggests that the true P3 composition corresponds to that observed in Orgueil and that the noble gas element varia-tions in other meteorites are in some way related to their release along the damage tracks which were produced during implantation of the ions in the first place. In this case we basically follow the results of [3] who showed that a bimodal noble gas release is produced as a result of a single im-plantation event, though our interpretation of the observation is quite different from that presented in [3]. We also suggests that only tracks produced by P3 gases have remained in the diamond struc-ture, since during implantation of HL gases the temperature was high enough to anneal defects associated with these gases. Tracks created by different noble gases are not equal in terms of the damage they produce in diamond structure. For instance, calculations us-ing TRIM code [4] indicate that for a diamond target implantation of noble gas ions with the same energy (1.2 keV) produces a higher number of vacancies in tracks formed by Xe and Kr com-pared to those for Ar (1.2x in a surface layer of ~8 Å thickness). This difference could, in fact, be even higher if implantation energy of Xe and Kr was higher than that for Ar. This seems to be quite a real possibility since stepped combustion results indicate that surface concentration of Ar in nanodiamonds is higher than that for Xe [5]. Thermal metamorphism and/or aqueous altera-tions could lead to a further disparity in the track defects associated with different noble gases. We suggest that noble gases of the P3-L com-ponent are associated with the most defective tracks. As tracks containing Ar are relatively less defective than those with Kr and Xe, then Ar should be released during thermal metamorphism or laboratory heating less effectively than Xe and Kr. Thus, the element variations shown in Figure 1 can be explained. Preferential loss of Kr over Xe could be connected with diffusion effects, if tracks associated with those gases are equally de-fective. Noble gases in the less damaged tracks are thought to represent the P3-H component. From thermally unmetamorphosed meteorites like Orgueil these gases are released during laboratory stepped heating only when the diamond structure degrades. Therefore thir release temperature coin-cides with that for HL gases. Thermal metamor-phism and aqueous alteration lead to a modifica-tion of the tracks resulting in a decrease of the release temperature of P3-H gases. References [1] Huss G. R and Lewis R. S. (1994) Meteoritics, 29, 794-829. [2] Verchovsky A. B. et al. (2000) J. Conf. Abstr.  (Goldschmidt 2000) 5, CDROM abstr. 1050. [3] Koscheev et al. (2001) Nature 412, 615-617. [4] (2003) www.srim.org. [5] Verchovsky A. B. at  al. (1998) Science 281, 1165-1168.  00.511.522.533.540 100 200 300 400 500 600 700 800 900P3-LP3-HPure end-membersΣP384Kr/132Xe36Ar/132XeHLP3OrLAKTVIBMS Figure 1. Element variation in P3 component in meteorites analysed in [1]. The letters corre-spond to the meteorites names. 00.511.522.533.540 100 200 300 400 500 600P3HLOrLAKTVIBMS84Kr/132Xe36Ar/132XeFigure 2. Element variation in the total gas release (HL+P3) up to 800oC in the same meteorites shown in Fig. 1.  Lunar and Planetary Science XXXV (2004) 1665.pdf

On the variations of the elemental composition of the P3 component in presolar diamonds

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On the variations of the elemental composition of the P3 component in presolar diamonds

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