Abstract-We studied the metallography of Fe-Ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of P-free, Fe-Ni alloy cooling experiments (described in Reisener and Goldstein 2003). Two types of Fe-Ni metal particles were observed in the chondrites: zoned taenite + kamacite particles and zoneless plessite particles, which lack systematic Ni zoning and consist of tetrataenite in a kamacite matrix. Both types of metal particles formed during metamorphism in a parent body from homogeneous, P-poor taenite grains. The phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles. Polycrystalline taenite particles transformed to zoned taenite + kamacite particles by kamacite nucleation at taenite/taenite grain boundaries during cooling. Monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process. The varying proportions of zoned taenite + kamacite particles and zoneless plessite particles in types 4-6 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism. Type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles. Type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles. The distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of Fe-Ni alloy phase transformations during cooling. The distribution cannot be explained by hot accretion-autometamorphism, post-metamorphic brecciation, or shock processing.

Abstract-Cooling rate experiments were performed on P-free Fe-Ni alloys that are compositionally similar to ordinary chondrite metal to study the taenite → taenite + kamacite reaction. The role of taenite grain boundaries and the effect of adding Co and S to Fe-Ni alloys were investigated. In P-free alloys, kamacite nucleates at taenite/taenite grain boundaries, taenite triple junctions, and taenite grain corners. Grain boundary diffusion enables growth of kamacite grain boundary precipitates into one of the parent taenite grains. Likely, grain boundary nucleation and grain boundary diffusion are the applicable mechanisms for the development of the microstructure of much of the metal in ordinary chondrites. No intragranular (matrix) kamacite precipitates are observed in P-free Fe-Ni alloys. The absence of intragranular kamacite indicates that P-free, monocrystalline taenite particles will transform to martensite upon cooling. This transformation process could explain the metallography of zoneless plessite particles observed in H and L chondrites. In P-bearing Fe-Ni alloys and iron meteorites, kamacite precipitates can nucleate both on taenite grain boundaries and intragranularly as Widmanstätten kamacite plates. Therefore, P-free chondritic metal and P-bearing iron meteorite/ pallasite metal are controlled by different chemical systems and different types of taenite transformation processes.

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Abstract-Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned taenite + kamacite particles (formed after relatively small amounts of taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after 200 °C of taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of taenite undercooling at temperatures <700 °C. The relevant silicate-metal reactions are modeled using chemical thermodynamics.Systematic olivine Fe-Mg zoning adjacent to metal is an expected consequence of retrograde silicate-metal reactions, and the presence of such zoning provides strong evidence that the silicate and metallic minerals evolved in situ during cooling from the peak metamorphic temperature.