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

Iron–nickel metallic particles have been reported in meteorites 1 and lunar 2–5 and terrestrial 6,7 rocks. The origin of these metallic particles is not unique as they may be formed by (1) condensation from a primordial solar nebula 8 ; (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 (Fo 92 ) 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, Fo 92 →Fo 96 +metallic Fe(Ni in solid solution)+pyroxene, begins with a homogeneous transformation involving fine-scale metallic precipitates resembling Guinier–Preston zones 9 . The microstructure develops by the growth of the first-formed precipitates during an Ostwald ripening process 9 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 materials 2 , these results have important implications for the physical models of accretion of terrestrial planets, planetesimals and meteorites 10 , 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.