Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.590v&link_type=abstract
Meteoritics, vol. 30, no. 5, page 590
Other
2
Allende, Diamonds, Efremovka, Grains, Presolar, Isotopes, Ar, Meteorites, Release Pattern, Orgueil
Scientific paper
Since it became clear that presolar diamonds are not identical and perhaps consist of several carriers of isotopically anomalous (like HL-Xe and light nitrogen) and isotopically normal (carbon and P3 noble gases) components, it is important to know how these components are related to the diamond carriers. Because of diamond grains are too small to be analyzed individually and the attempts to separate the diamonds [1] have had little, if any, success, the only method currently available to investigate the relationship between the carriers and corresponding components is the temperature dependence of their release pattern. The interpretation of the latter however is often not straight forward because the mechanism by which the gases are released is complicated and simultaneously involves several processes such as diffusion, chemical reactions between coexisting phases and/or their heating environment and phase transformations [2, 3]. To unravel this interdependent matrix we use several extraction methods applied to the same sample. In the present study we used a combination of stepped pyrolysis and combustion for investigation of the release pattern of Ar components for presolar diamonds concentrated from CI (Orgueil), and CV (Allende, Efremovka) meteorites. From the previous studies [2] it is known that Ar in the most primitive CI's and CM's meteorites consists mostly of P3 component while for CV3's like Allende and Efremovka it is represented mainly by HL component. This difference in the noble gas components composition probably reflects the thermal history of the corresponding parent bodies and the extreme sensitivity of the P3 component concentrations, in presolar diamonds, to the thermal metamorphism. Laboratory pyrolysis experiments show that most of the P3 argon is released at rather low temperature though some of the gas is still being liberated at temperatures as high as those usually associated with HL component [2]. The low release temperatures have led to the idea that P3 noble gases are located near the surface of the diamond grains which is destroyed by pyrolytically induced chemical reactions with surface bonded H atoms [2]. However the same authors did not comment on the high temperature part of the P3 release. Our previous investigations [4] however have clearly showed that the release of P3 component during pyrolysis is accompanied by of 8-15% of C present in the form of CO+C02. This observation is in agreement with the amount of oxygen which can be adsorbed on the surface of diamond grains. The evolution of C02+CO is in any case independent of whether or not external oxygen is used for the extraction. Analysis of a high resolution release pattern of 36Ar from Orgueil shows (Fig.) that it is bimodal and similar for pyrolysis and combustion. There is a fraction of P3 Ar (~40%) which is released at low temperature apparently by diffusion before the carbon begins to combust. The rest of the P3 Ar in the case of combustion, and another 40 - 50% in the case of pyrolysis, is released in close association with release of carbon. We conclude that the P3 component has a complicated nature and some difference in the isotopic composition of its constituents may be suggested. It is important to note that release of Ar P3 occurs over the whole temperature range of diamond combustion (450-600 degrees C) independent of how much oxygen is available (Fig.). This is evidence against a location of P3 gases on the surface of the diamond grains but suggests a separate camer for the component. The release of about 10% of P3 gases at very high temperature during pyrolysis [2] can be simply explained if the amount of the oxygen associated with the sample is not enough to oxidize all the P3-containing grains. The release of P3 gases together with HL component contradicts their surface location. We believe these is now a strong case for P3 gas (Ar at least) in Orgueil being associated with a distinct type of grain which constitutes ~10% by weight of the carbon. In contrast to the above a high resolution stepped combustion extractions of Ar from Allende and Efremovka indicate a very close correlation between 36Ar and carbon leaving no doubt about location of HL component in the diamonds. References: [1] Lewis R. S. (1994) LPS XXV, 793. [2] Huss G. R. and Lewis R. S. (1994) Meteoritics, 29, 791. [3] Verchovsky A. B. and Pillinger C. T. (1994) Meteoritics, 29, 543. [4] Verchovsky A. B. et al. (1993) Meteoritics, 28, 452.
Fisenko Anatolii V.
Pillinger Colin T.
Verchovsky Alexander B.
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