Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXIII, No.2

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Texture, Chemical Composition and Genesis of Schreibersite in Iron Meteorite

Yoshikawa, Masahide;Matsueda, Hiroharu

Permalink : http://hdl.handle.net/2115/36782
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Abstract

Thirteen iron meteorites composed of Hexahedrite, Octahedrite and Ataxite were investigated to estimate their cooling history and origin. They are mainly composed of Fe-Ni metals (kamacite and taenite) with smaller amounts of schreibersite (Fe, Ni)3P and sulfides (troilite and sphalerite). Schreibersite occurs an idiomorphic and xenomorphic crystals and its mode of occurrence is variable in iron meteorites. Xenomorphic schreibersite is subdivided into 6 types on the basis of their textures and relationships with coexisting minerals. Chemical composition of schreibersite varies from 20 to 40 atom. % Ni with textural types among some iron meteorites with different bulk chemical compositions and even in the same meteorite (e.g. Canyon Diablo), while it does not vary so clear with textural types in ALH-77263. Schreibersite seems to maintain a local equilibrium with coexisting metal phases. Based on the Fe-Ni-P phase diagram, it is estimated that xenomorphic and coarse-grained schreibersite in Y-75031 and in DRPA 78007 were crystallized from stability field of taenite and schreibersite at about 800℃ under rapid diffusion conditions. Cooling rates within the parent bodies are calculated by using the diffusion rate of Ni between schreibersite and kamacite as follows; Canyon Diablo: 1.1×10^6, Odessa: 1.7×10^6, North Chile: 2.3×10^6, ALH-77263: 8.7×10^7 (year/℃). It is considered that ALH-77263 had been cooled keeping more equilibrated state with slower cooling rate than Canyon Diablo. Variable textures and chemical compositions of schreibersite suggest that it had been maintained a local equilibrium with kamacite. Estimated temperatures at which the reaction mostly completed among metals and sulfide tend to show higher value in unequilibrated iron meteorites than equilibrated ones. It is inferred that unequilibrated iron meteorites had been rapidly cooled, but equilibrated ones slowly to maintain diffusion under the lower temperatures.

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