Siderophile elements and metal-silicate fractionation in the solar nebula
The most important nebular fractionation affecting the siderophile elements is the metal-silicate fractionation process and its relationship to the chondrule formation process is poorly understood. Understanding these processes is important in terms of understanding the expected compositional range for planetary building blocks. In a general way the composition of chondrites can be derived from the composition of the CI chondrites by addition or subtraction of a refractory component similar to CAI's (in some cases with a Mg2SiO4 component) and by addition or subtraction of Fe metal. Thus normalization to Fe produces the least spread in the normalized abundances of most siderophile elements relative to CI abundances. Detailed bulk chemical studies of chondrules have shown that their siderophile elements have refractory-element siderophile-element signatures (for example, Ir/Ni) that are similar to the bulk meteorites, but distinct among the different chondrite groups. This data suggests that the chondrules were not supplied to the chondrule groups from a single homogeneous source, and that each chondrite group's characteristic siderophile-element signature was established before chondrule formation. These authors make a further inference that all siderophile-lithophile-element fractionation occurred before chondrule formation, but recent discoveries and observations suggest this is not true. The discovery of the metal-rich CH meteorites, the recognition of the role of aerodynamic forces in the solar nebula, and the possible role of metal-silicate segregation during the chondrule formation process suggests that metal-silicate fractionation occurred before, during and after chondrule formation.