Isotopic anomalies of noble gases in meteorites and their origins: 2. Separated minerals from Allende

Abstract

Eleven mineral fractions from the Allende C3 chondrite were analyzed for He, Ne, Ar, Kr, and Xe by mass spectrometry. As was shown previously (Lewis et al., 1975), most of the heavy noble gases, of unexceptional isotopic composition, are contained in an ill‐defined HNO3‐soIuble mineral fraction (‘Q’) comprising 0.04% of the meteorite. The remaining noble gases, of decidedly nonsolar isotopic composition, are contained in chromite (0.2%) and amorphous carbon (0.2%). They have virtually identical concentrations and isotopic compositions in the two minerals: 20Ne/22Ne = 8.7 and 36Ar/38Ar = 4.82. Xenon is enriched up to twofold in both heavy and light isotopes, while Kr is enriched only in heavy isotopes. The enrichment of the heavy isotopes is presumably due to fission, but the fission spectra derived depend critically on the assumed composition of the trapped component. The enrichment patterns of the light xenon isotopes differ slightly for chromite and carbon. Partial destruction of the minerals by chemical treatments gave no evidence for other gas‐rich phases or other gas components and only orginal evidence for heterogeneities within the known gas‐rich phases. Spinel has ≪0.1 the gas content of other phases. The chromite‐carbon fractions contain 30–60% more spallogenic 21Ne than does the bulk meteorite, although they are deficient in target elements for the production of 21Ne. A possible explanation is recoil of 21Ne from adjacent Mg‐rich phases. All of the available isotopic and chemical data suggest that chromite, carbon, and Q, and hence their associated gas components, are of local rather than extra‐solar system origin. The excess heavy Kr and Xe isotopes may have been produced by fission of a superheavy element, while the remaining isotopic anomalies may be due to mass fractionation during trapping. The virtual identity of the noble gas components of chromite and carbon suggests that both minerals formed simultaneously, perhaps by a reaction such as Fe + 2Cr + 4CO → FeCr2O4 + 4C.

Spallation xenon from mesosiderites.

“…Another com ponent of Xe with an unusual isotopic yield pattern has been found in some carbonaceous chondrites, and has been attributed at times to fission. Arguments supporting a super-heavy element progenitor for this component continue to be presented by ANDERS and his coworkers (ANDERS and HEYMANN, 1969;LEWIS et al, 1977aLEWIS et al, , 1977bSRINIVASAN et al, 1978 and references therein) in spite of firm opposition from MANUEL and others (MANUEL, 1977;MANUEL and SABU, 1977, and references therein) who argue that it is produced by nucleosynthetic processes.…”

Elemental and isotopic heterogeneities of rare gases in the solar system: Reply to a comment by Kuroda and Sheng

“…This can be seen, for example, in Figure 2 of Lewis et al (1977) and in Figure 3 of Ott et al (1984). Values of the 136Xe/132Xe isotopic ratio compared with the elemental ratios of He and Ne rela tive to Xe, Kr and Ar in acid residues of Allende (Lewis et al ., 1975 Why does Figure 1 of Kuroda and Sheng (1987) show a scatter from the correlation lines for rare gases in bulk Allende and in the temperature fractions of the Allende 3CS2 residue?…”

Composition of the noble gases in Canyon Diablo.

“…4 for Xe. The isotopic compositions of Kr and Xe in Allende residue 4C57 (LEwis et al, 1977) are shown as examples of gases that are predominantly type-X, and the isotopic com positions of these gases in air (NIER, 1950b;NIEF, 1960) and in the Kenna ureilite (WILKENING and MARTI, 1976) . Three isotope plot of Kr indicating the indig enous gas component in Canyon Diablo.…”