Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.327b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 327
Other
Allende, C-Delta, Diamonds, Interstellar Grains, Noble Gases
Scientific paper
We have performed a high-resolution step-wise pyrolysis experiment (29 temperature steps from 600 degrees C to 2100 degrees C) on a 1.625 mg sample of diamond (is identical to C delta) residue from Allende (CV3) prepared at Washington University. Our initial motivation for this work was to determine, by utilizing a relatively large number of release fractions, whether any of the Xe components previously observed in meteoritic C delta residues (e.g., Xe-P3, Xe-HL is identical to Xe-Ex, Xe-Y [1-4]) could be better resolved. In particular, we wanted to determine whether any temperature-dependent separation of Xe-H from Xe-L could be observed (as has been noted by [2]). However, as expected on a priori grounds [e.g. 4], such a separation was not observed, and the Ne, Ar, Kr, and Xe analyses of this work are consistent, within error, with previous noble gas analyses of meteoritic diamond [1-4]. The Xe data (Fig. 1) show the three-component mixing seen in earlier works [1-4]. The inferred Xe-Ex endmember composition is in good agreement with the results of [4], which utilized a larger suite of meteorites. The inferred limits on the possible endmember compositions of the Xe-Y component, however, are somewhat different from those derived in [4] (See Fig. 1). The Ar data show a similar three-component mixing (Ar-P3, Ar-Ex, Ar-Y; cf. [4]), and we obtain the following limits for ^38Ar/^36Ar in the latter two components: (^38Ar/^36Ar)(sub)Ex > 0.2220 and (^38Ar/^36Ar)(sub)Y < 0.2088. In addition, our data indicate that (Ar/Xe)(sub)P3>(Ar/Xe)(sub)Y>(Ar/Xe)(sub)Ex, consistent with the results of [4]. The Ne data clearly indicate the presence of at least two components: Ne-P3 and Ne-A2. On a standard Ne three- isotope plot the Ne data lie within ~2 sigma of a line connecting the points (20:21:22) = (8.45:0.032:1.00) and = (7.65:0.41:1.00). Although Ne-A2 may possibly represent a mix between a Ne-Ex and a Ne-Y component [4], we cannot resolve Ne-Ex from Ne-Y, neither on the basis of the Ne isotopes alone nor by relying on the elemental abundances of these components for Ar and Xe. Contributions from a Ne-E-like component and spallogenic Ne may be required to explain the data. However, such contributions are probably not indigenous to the diamonds, but may be due to minor contaminant phases or to gases inherited from phases dissolved during processing. We also observe clear excesses of spallogenic and halogen-derived (from neutron captures) Ar, Kr, and Xe in the lower temperature fractions. Again, these components are probably not indigenous to the diamonds, but were presumably acquired by the diamonds from other phases during processing or via implantation from spallation recoils during Allende's recent cosmic ray exposure. Similar neutron effects occurring during Allende's recent exposure have been observed by [5] and larger neutron effects were observed in an Inman residue of lesser purity [2]. References. [1] Lewis R. S. and Anders E. (1988) LPS XIX, 679- 680. [2] Nichols R. H. Jr. et al. (1991) GCA, 55, 2921-2936. [3] Verchovsky et al. (1993) LPS XXIII, 1467-1468. [4] Huss G. R. and Lewis R. S. (1993) Meteoritics, submitted. [5] Gobel R. et al. (1992) GCA 46, 1777-1792.
Brazzle Robert Harald
Gao XingHui
Hohenberg Charles M.
Nichols Robert Hill Jr.
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