Brecciation and chemical heterogeneities of CI
chondrites
Fragments in the size range from 40 µm to several hundred µm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk show a wide range of chemical compositions with variations in major elements such as iron (10.4&endash;42.4 wt.% FeO), silicon (12.7&endash;42.2 wt.% SiO2) and sulfur (1.01&endash;15.8 wt.% SO3), but also important minor elements such as phosphorus (up to 5.2 wt.% P2O5) or calcium (up to 6.6 wt.% CaO). These variations are the result of the varying mineralogical compositions of these fragments. The distribution of phyllosilicates, magnetites, and possibly ferrihydrite, in particular, control the abundances of these elements. High REE contents&endash;up to 150 times the solar abundances&endash;were observed in phosphates, while matrix and sulfates are REE-depleted. The studied 113 fragments were subdivided into eight lithologies with similar mineralogical and, thus, chemical properties. The most common is the CGA lithology, consisting of a groundmass of Mg-rich, coarse-grained phyllosilicates and varying abundances of inclusions like magnetite. The second most abundant lithology is the FGA lithology, consisting of a groundmass of fine-grained Fe-rich phyllosilicates. A rare, but important, lithology consists of fragments with high contents of phosphates and other minerals. The proposed model for the evolution of these lithologies is based on a closed system, where mineralogical differences in the lithologies reflect heterogeneities in the starting material. Comparison of our results with literature data indicates a general similarity of the four CI chondrites analyzed. Further comparison of bulk analyses suggests that the mass "threshold" for chemical heterogeneities in CI chondrite samples is smaller than ~1&endash;2 g.
Morlok et al. (2006) Geochim. Cosmochim. Acta 70, 5371-5394.
Full publication: 2006_GCA70_5371.pdf
Last revised: 11-2006