Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.519p&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 519
Other
3
Anomalies, Carbonaceous Chondrites, Chromium Isotopes, Phase Transformations, Dissolving, Mapping, Scanning Electron Microscopy, Solubility
Scientific paper
Carbonaceous chondrites, especially CI chondrites such as Orgueil, have endemic isotopic anomalies in Cr distributed in a manner uncommon in comparison with other known occurrences of isotopic anomalies. As revealed in sequential dissolution analyses, all the Cr, at least in CIs such as Orgueil, is anomalous. The most easily dissolved (in acetic and nitric acids) material, evidently including alteration phases, is deficient in Cr-54 by a few epsilon-units; material dissolving in HCl has Cr-54 excesses of several permil; phases that dissolve only in more aggressive reagents have smaller excesses of Cr-54. Mass balance indicates that total Cr is approximately normal. As yet there are no known correlated anomalies in other elements. The simplest hypothesis consistent with these observations is that there is (at least) one phase, presently not identified, that is significant in the Cr mass balance and that contains at least about 1% excess Cr-54. This phase is mechanically admixed in approximately the right proportion to produce normal composition, but has remained chemically and isotopically distinct through nebular and parent body processing. It must be enriched in Cr, at least in comparison with other elements so far known to lack parallel anomalies. Unlike known kinds of interstellar grains or prominent anomaly-bearing minerals such as hibonite or spinel, it is not resistant to HCl/HF. Most of the Cr in Orgueil is in more readily soluble phases and has a Cr-54 deficit because it has not mixed with its 'cosmic complement' of Cr-54 in the unknown phase.
Alexander Clark
Bernatowicz Thomas J.
Brannon Joyce C.
MacPherson Glenn J.
Neal Chris R.
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