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 com plex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide-rich, composed primarily of troilite and containing abu ndant embedded silicates; (2) nonchondritic, silicate-rich, comprised of ba saltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralo gy and most closely resembling the winonaites in composition and texture; ( 4) rounded, often graphite-rich assemblages that sometimes contain silicate s; 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 c hondrite group. Metamorphism, partial melting,and incomplete differentiatio n (i.e., incomplete separation of melt from residue) produced metallic, sul fide-rich and silicate partial melts (portions of which may have crystalliz ed prior to the mixing event), as well as metamorphosed chondritic material s and residues. Catastrophic impact breakup and reassembly of the debris wh ile 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 impac t 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 G a indicate this entire process occurred early in solar system history.