Origin of iron meteorites - the oldest materials in the Solar System

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Models of the formation of magmatic iron meteorites assume gravitational segregation of liquid metal from a silicate melt. The Hf-W dating method determines the timing of this event and shows that iron meteorites have about the same age as Ca,Al-rich inclusions (CAI), the oldest rocks in the solar system but are older than chondrules the major component of chondrites. This age relationship is in contrast to the general assumption that differentiated bodies are the product of melting of chondritic parent bodies. It emphasizes the importance of magmatic iron meteorites which represent the metal cores of the oldest planetesimals. Similarly old silicate-rich material does not exist. The former silicate-rich mantles of the iron meteorites may be reconstructed i) by studying non metal inclusions that could have formed by exsolution and ii) by direct analyses of lithophile elements, like Si, Al, Cr, and P in metal phases. The concentrations of these elements are inconsistent with metal/silicate melt equilibrium at high temperature but reflect low temperature processes. The process responsible for loosing lithophile elements from the metal was most efficient in IIIAB iron meteorites which have extremely low concentrations of such elements. Inclusions of silicates, oxides, phosphates, phosphides or sulfides in iron meteorites may have acted as sink for these elements. We will continue to analyze metal phases of group IC, IIC and especially IVA and IVB magmatic irons which have the highest concentrations of Si and Al in all studied iron meteorites. We will also continue to search for non-metal inclusions in these meteorites. In addition, oxygen isotopes of inclusions will be measured to provide information about the isotopic signature of the parent body, degree of homogenization and origin of the oxygen.