Iron and Stony-Iron Meteorites: Evidence for the Formation, Crystallization, and Early Impact Histories of Differentiated Planetesimals

By far most of the melted and differentiated planetesimals that have been sampled as meteorites are metal-rich iron meteorites or stony iron meteorites. The parent asteroids of these meteorites accreted early and differentiated shortly after the solar system formed, producing some of the oldest dated materials. The main heat source responsible for the melting and differentiation of asteroids was 26Al (Chapter 6, This Volume). Unlike the parent bodies of chondrites, the differentiated bodies accreted while 26Al was sufficiently abundant to cause melting. In this review, we summarize properties and interpretations for iron and stony iron meteorites (Plate 13.1). Such meteorites provide important constraints on the nature of metal-silicate separation and mixing in planetesimals undergoing partial to complete differentiation. They include iron meteorites that formed by the solidification of cores (fractionally crystallized irons), irons in which partly molten metal and silicates of diverse types were mixed together (silicate-bearing irons), stony irons in which partly molten metal and olivine from cores and mantles were mixed together (pallasites), and stony irons in which partly molten metal and silicate from cores and crusts were mixed together (mesosiderites