A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB-Winonaite parent body

We studied 26 IAB iron meteorites containing silicate-bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide-rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate-rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite-rich assemblages that sometimes contain silicates; and (5) phosphate-bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and O-isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body. We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting,and incomplete differentiation (i.e., incomplete separation of melt from residue) produced metallic, sulfide-rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials from various depths into the re-accreted parent body. Thus, molten metal from depth was mixed with near-surface silicate rock, resulting in the formation of silicate-rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate-free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4.40-4.54 Ga indicate this entire process occurred early in solar system history.