Siderophile elements and molybdenum, tungsten, and osmium isotopes as tracers of planetary genetics and differentiation: Implications for the IAB iron meteorite complex

Abstract

Isotopic and trace element abundance data for iron meteorites and chondrites are presented in order to investigate the nature of genetic relations between and among meteorite groups. Nebular and planetary diversity and processes, such as differentiation, can be better understood through study of IAB complex iron meteorites. This is a large group of chemically and texturally similar meteorites that likely represent metals with a thermal history unlike most other iron meteorite groups, which sample the cores of differentiated planetesimals. The IAB complex contains a number of chemical subgroups. Trace element determination and modeling, Re/Os isotopic systematics, nucleosynthetic Mo isotopic data, and 182Hf-182W geochronology are used to determine the crystallization history, genetics, and relative metal-silicate segregation ages of the IAB iron meteorite complex. Highly siderophile element abundances in IAB complex meteorites demonstrate that diverse crystallization mechanisms are represented in the IAB complex. Relative abundances of volatile siderophile elements also suggest late condensation of some IAB precursor materials. Improvements in the procedures for the separation, purification, and high-precision analysis of Mo have led to a ~2 fold increase in the precision of 97Mo/96Mo isotope ratio measurements, compared to previously published methods. Cosmic ray exposure-corrected Mo isotopic compositions of IAB complex irons indicate that at least three parent bodies are represented in the complex. The Hf-W metal-silicate segregation model ages of IAB complex subgroups suggests that at least four metal segregation events occurred among the various IAB parent bodies. The IAB complex samples reservoirs that were isotopically identical, but chemically distinct, and reservoirs that were chemically similar in some respects, yet isotopically different. Some IAB subgroups are genetically distinct from all other iron meteorite groups, and are the closest genetic relations to the Earth. The chemical differences between magmatic iron meteorite groups and some IAB subgroups likely originated as a result of their formation on undifferentiated parent bodies where impacting and mixing processes were important. This, in addition to the genetic difference between some IAB irons and magmatic groups, implies that these IAB parent bodies and magmatic parent bodies formed in a different location and/or time in the protoplanetary disk.