Hunting for the Carrier of a New Xe Component (H-C(sub)delta Xe) in Diamond-rich Residues of Primitive Meteorites

Details Hunting for the Carrier of a New Xe Component (H-C(sub)delta Xe) in Diamond-rich Residues of Primitive Meteorites Hunting for the Carrier of a New Xe Component (H-C(sub)delta Xe) in Diamond-rich Residues of Primitive Meteorites

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..301v&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 301

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Scientific paper

Noble gases released at high temperature during both stepped pyrolysis and combustion of diamond-rich residues separated from some primitive meteorites (CV3s-Allende, Efremovka; LL3.4-Inman) contain Xe with isotopic composition different from HL-, Sol-, Pl- and S-Xe or their mix (Lewis and Anders, 1988; Nichols et al., 1991; Verchovsky et al., 1992). The regular recognition of the new Xe component or subcomponent requires it be given a name; we suggest to call it high-temperature Cdelta xenon or H-Cdelta Xe. At present only few characteristic features of the component and its possible host phase can be pointed out: Firstly, its occurrence only in association with what are believed to be very pure diamond separates hosting Xe-HL, characterised by ^136Xe/^132Xe >0.6, means that the carrier of the H-Cdelta Xe is very resistant to oxidising acids. Secondly, release of the H-Cdelta Xe in the range 600-800 degrees C in combustion experiments (Verchovsky et al., 1992) and at T > 1300 degrees C in pyrolysis experiments (Lewis and Anders, 1988; Nichols et al., 1991) suggests a host phase that is more resistant to pure oxygen and retentive with respect to the release of noble gases by diffusion than nanometer sized diamonds. It should be noted that stepped combustion provides significantly better resolution of the noble gas components than stepped pyrolysis and particularly at high temperature where for the former there is a large temperature gap between the end of Cdelta diamond combustion (~500 degrees C) and the majority SiC burning (~1000 degrees C); during pyrolysis the evolution of Xe-HL and Xe-S frequently overlap. Thirdly, the H-Cdelta Xe is accompanied by other noble gases in particular by Ne with ^20Ne/^22Ne lower than in NeA2 (Verchovsky et al., 1992). Based on these observations the H-Cdelta Xe and the associated noble gases are likely to be of presolar origin and their carrier appears to be a carbon phase. Indeed all the residues from the meteorites listed above contain a carbon phase combusting at 600-800 degrees C with delta^13C in the range from -20o/oo to +20o/oo. The abundance of the phase is relatively low (~1000 ppm in the residue) though noble gas concentrations in this minor phase are higher than for those in Cdelta. A high resolution TEM investigation of separates from Murchison has demonstrated (Gilkes et al., 1992) the presence of minor components other than diamond including carbynes, which could be regarded as candidates for the carrier of H-Cdelta Xe. With a better knowledge of the carbon abundance and delta^13C characteristics of the potential carrier of H-Cdelta Xe we have reconsidered data both published and unpublished for diamond-rich samples produced in our laboratory and have selected a number of candidate residues that may help in its characterisation. In particular, samples isolated from Allende and Krymka seem most promising. A diamond-rich separate from Krymka (Fisenko et al., 1992) indeed contains the carbon phase combusting at 600-800 degrees C with delta^13C ~ +10o/oo in particularly high abundance. Noble gas data for the candidate samples are being acquired. References: Fisenko A.V., Russell S.S., Ash R.D., Semjonova L.F., Verchovsky A.B. and Pillinger C.T. (1992) Lunar Planet. Sci. (abstract) 23, 365; Gilkes K.W.R., Gaskell P.H., Pillinger C.T., Russell S.S. and Arden J.W. (1992) Meteoritics, this volume; Lewis R.S. and Anders E. (1988) Lunar Planet. Sci. (abstract) 19, 679; Nichols R.H. Jr., Hohenberg C.M., Alexander C.M.O'D., Olinger C.T. and Arden J.W. (1991) Geochim. Cosmochim. Acta 55, 2921; Verchovsky A.B., Ott U., Russell S.S. and Pillinger C.T. (1992) Meteoritics 26, 402.

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