Cr in CI meteorites displays isotopic anomalies in a unique pattern [1-3]. Etching of bulk Orgueil in relatively mild reactants (CH3COOH and dilute HNO3) dissolves most (>80%) of the Cr along with most of other major cations (Mg, Ca, Fe, Ni, Al). The Cr thus dissolved is deficient in 54Cr by about 6 epsilon-units (using 50Cr/52Cr normalization). Much of the remaining Cr is released in further etching in HCl; this Cr has stronger excesses of 54Cr, up to around 100epsilon. Dissolution of the residues in more aggressive reactants yields Cr with smaller excesses of 54Cr. There are no known correlating anomalies in other elements or even in 53Cr. Although no Cr carrier in Orgueil has isotopically normal Cr, mass balance calculations indicate approximately normal Cr in the whole rock. We have previously suggested that these results could be most simply explained by postulating one carrier phase, relatively rich in Cr and bearing strong excess 54Cr, presumably from neutron-rich nuclear statistical equilibrium nucleosynthesis. This phase would have to be soluble in HCl but resistant to parent body aqueous alteration. In this interpretation the 54Cr-deficiency in most of the Cr is understandable in terms of mixing of all the nucleosnthetic sources contributing to solar normal except for that in the postulated carrier phase. Qualitatively similar but more subdued isotopic effects occur in CM meteorites and are very subdued or absent in CV and CO meteorites. If the parent bodies for these classes originally contained the same postulated phase in comparable abundance, the implication is that the 54Cr-enriched carrier phase is also not resistant to the thermal processing experienced by these classes. Interstellar grains bearing Cr from specific nucleosynthetic events would be expected to be much more anomalous than any samples yet observed; we thus lack evidence to determine whether the carrier is itself a type of interstellar grain (in which case all observations to date reflect major dilution with more nearly normal Cr) or whether it is a solar nebula product preferentially incorporating such interstellar grains. In either case, the implicated interstellar grain type is clearly different from the types already known and isolated (diamond, graphite, carbides, oxides), all of which are chemically and thermally resistant to nebular processing. We have examined fractions of Orgueil separated from a CH3COOH/HNO3 residue by a teflon-coated magnetic stirring bar. For both fractions HCl treatment yields Cr more strongly enriched in 54Cr (by about 200epsilon) than any previously observed. Evidently this procedure is ineffective in separating the carrier (or there is more than one carrier); the magnetite which comprises most of the magnetic fraction is apparently just a dilutant. SEM/EDX analysis of the various etch residues (and untreated bulk) for Orgueil reveals the presence of small (mostly submicron) Cr-rich grains (Cr and O, often with significant Fe and/or Mn). Quantitative abundance determination of such grains is difficult, nor can we be confident about their disappearance or persistence in HCl etching, but such grains could account for the observed isotopic effects if they are very enriched in 54Cr. Preliminary ion probe measurements on the non-magnetic fraction of Orgueil indicate that most Cr-rich spots have normal Cr within errors but that some have large 54Cr enrichments (at least tens of percent). One such grain may have a truly radical Cr isotopic composition. References:  Rotaru M. et al. (1992) Nature, 358, 465-470.  Ott U. et al. (1994) LPSC XXV, 1033-1034.  Podosek F. A. et al. (1994) Meteoritics, 29, 519.
- Pub Date:
- September 1995