CARBIDE-MAGNETITE ASSEMBLAGES IN TYPE-3 ORDINARY CHONDRITES

Abstract

Abundant carbide-magnetite assemblages occur in matrix, chondrules, and chondrule rims in several H3, L3, and LL3 chondrites. Carbides, cohenite ((Fe,Ni)3C), and haxonite ((Fe,Ni)23C6) show compositional variations between different meteorites and appreciable ranges within meteorites. 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 with 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 observations indicate that carbide-magnetite assemblages formed by replacement of metal-sulfide nodules. The high Co contents of residual kamacite in association with carbides indicates that Co is not incorporated into carbides (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 residual taenite tend to be high. Ni-rich sulfides were found only in LL3 chondrites, possibly indicating their more extensive oxidation and/or aqueous alteration.We suggest that carbide-magnetite assemblages in type-3 ordinary chondrites formed as the result of hydrothermal alteration 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) = Fe3C(s) + Fe3O4(s) or 3 Fe(s) + 2 CO(g) = Fe3C(s) + CO2(g)), and oxidation, probably by H2O gas (e.g., 3Fe(s) + 4H2O(g) = Fe3O4(s) + 4H2(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 result of the reaction of carbon compounds (e.g., hydrocarbons) with water vapor or magnetite.