CARBIDE-MAGNETITE ASSEMBLAGES IN TYPE-3 ORDINARY CHONDRITES

Abundant carbide-magnetite assemblages occur in matrix, chondrules, an d chondrule rims in several H3, L3, and LL3 chondrites. Carbides, cohe nite ((Fe,Ni)(3)C), and haxonite ((Fe,Ni)(23)C-6) show compositional v ariations between different meteorites and appreciable ranges within m eteorites. Carbides in H chondrites have lower Co contents (0-0.6 wt%) than those in L and LL chondrites (0.3-1.2 wt%). Metal associated wit h carbides and magnetite consists of high-Ni (50-70 wt%) taenite and, in L and LL chondrites, Co-rich (up to 35 wt%) kamacite; minor element contents of troilite and magnetite are very low. Textural observation s indicate that carbide-magnetite assemblages formed by replacement of metal-sulfide nodules. The high Co contents of residual kamacite in a ssociation with carbides indicates that Co is not incorporated into ca rbides (i.e., Fe/Co is much higher in the carbides than in kamacite). Because Ni in carbides and magnetite is low, the Ni contents of residu al taenite tend to be high. Ni-rich sulfides were found only in LL3 ch ondrites, possibly indicating their more extensive oxidation and/or aq ueous alteration. We suggest that carbide-magnetite assemblages in typ e-3 ordinary chondrites formed as the result of hydrothermal alteratio n of metallic Fe in metal-troilite nodules by a C-O-H-bearing fluid on their parent bodies. This alteration resulted in carbidization of Fe- Ni metal, probably by CO gas (e.g., 15 Fe(s) + 4 CO(g) = Fe-3(s) + Fe3 O4(s) or 3 Fe(s) + 2 CO(g) = Fe3C(s), CO2(g)), and oxidation, probably by H2O gas (e.g., 3Fe(s) + 4H(2)O(g) = Fe3O4(s) + 4H(2)(g)). The C-O- H-bearing fluids, which were possibly released during metamorphism and transported through zones of high permeability, may have been derived from ices, adsorbed gases, or hydrated minerals. The CO may be the re sult of the reaction of carbon compounds (e.g., hydrocarbons) with wat er vapor or magnetite. Copyright (C) 1997 Elsevier Science Ltd