Ellen R. Harju^a, Alan E. Rubin^b, Insu Ahn^c, Byeon-Gak Choi^d, Karen Ziegler^a,1, John T. Wasson^a,b,e

^aDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
^bInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
^cKorea Polar Research Institute, Incheon, 406-840, Korea
^dDepartment of Earth Science Education, Seoul National University, Seoul, 151-748, Korea
^eDepartment of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1567, USA

The wide range in the degree of aqueous alteration of CR chondrites prompted us to formulate a numerical sequence for these rocks that ranges from petrologic type 2.0 to 2.8. (Hypothetical CR3.0 chondrites should be completely free of aqueous alteration effects.) About 70% of CR chondrites are slightly altered, type-2.8 rocks that exhibit heterogeneous alteration; these meteorites contain moderately abundant metallic Fe-Ni, no magnetite, and generally, a few chondrules with clear glassy mesostases. None of the chondrules in these rocks shows evidence of alteration of mafic silicate phenocrysts, but several chondrules are surrounded by phyllosilicate-rich rims that appear “smooth” when viewed by back-scattered-electron imaging. Matrix regions in slightly altered CR chondrites contain high S (~3 wt.%), but some matrix patches in the same thin sections record alteration effects and contain appreciably less S (<1.5 wt.%). In CR chondrites that have been more-significantly altered (e.g., Renazzo and Al Rais), metallic Fe-Ni has been partially replaced by magnetite±sulfide; mafic silicates have been partly altered to phyllosilicates, particularly along edges, fractures and twin boundaries. One of the most-altered CR chondrites (type-2.0 GRO 95577) contains abundant magnetite, additional oxide phases, iron carbonate, only very rare metallic Fe-Ni and essentially no mafic silicate grains. The whole-rock O-isotopic compositions of CR chondrites correlate with the degree of aqueous alteration: Δ¹⁷O ranges from ~-2.6‰ in type-2.8 samples to ~-0.4‰ in type 2.0.

Reference
Harju ER, Rubin AE, Ahn I, Choi B-G, Ziegler K and Wasson JT (in press) Progressive aqueous alteration of CR carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.048]
Copyright Elsevier

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