Solid-state reduction of iron in olivine—planetary and meteoritic evolution

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Abstract
Iron-nickel metallic particles have been reported in meteorites1 and lunar2-5 and terrestrial6,7 rocks. The origin of these metallic particles is not unique as they may be formed by (1) condensation from a primordial solar nebula8; (2) crystallization from a melt; and (3) subsolidus reduction reactions under low oxygen or sulphur fugacity. We report here an electron microscopy study of the solid-state microstructural development in olivine single crystals (Fo92) in which half of the iron has been reduced to the metallic state by a gas-solid interaction in the temperature range 950-1,500 °C. The reaction, Fo92→Fo96+metallic Fe(Ni in solid solution)+pyroxene, begins with a homogeneous transformation involving fine-scale metallic precipitates resembling Guinier-Preston zones9. The microstructure develops by the growth of the first-formed precipitates during an Ostwald ripening process9 in which the precipitates located in the dislocation sub-boundaries develop in preference to precipitates in the subgrains. On the other hand, pyroxene is first observed to nucleate heterogeneously at pre-existing dislocations and its coarsening rate is more than an order-of-magnitude faster than that of the metallic phase. Besides the textural similarity of the observed microstructures with that reported for some of the lunar materials2, these results have important implications for the physical models of accretion of terrestrial planets, planetesimals and meteorites10, especially with respect to the distribution of siderophile elements. The rate of reaction observed here places constraints on models for the formation of the Earth's core by segregation of a metallic phase with or without reduction.

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