| Until the past decade, IIAB iron meteorites were thought to have formed by fractional crystallization under low pressure, corresponding to an asteroidal sized (<200 km) parent body. Some recent physical models for the formation of early terrestrial bodies propose that iron meteorites may have formed by the collisional disruption of larger planetesimal-sized (>1000 km) bodies where higher pressures would prevail in the core(s). To test models of fractional crystallization, 21 IIAB iron meteorites (and 2 replicates) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for their elemental compositions that provided new abundance data for Ru, Rh, Pd, Sn, Sb, and Os. Highly incompatible elements (e.g., As, Pd, Sn, Sb, Au) are useful tools for studying fractional crystallization in irons. Antimony has been experimentally shown to be one of the most incompatible elements present in irons. Thus, a primary objective was to obtain precise abundances for Sb to explore its usefulness for studying fractional crystallization. New analytical data in conjunction with previously published literature data are used to model fractional crystallization of IIAB irons as a function of the sulfur content in the melt fraction. A contradiction between the experimental partitioning models of As and Au with the IIAB iron data led to an investigation of the effects of elevated pressure acting on the IIAB core. Relating fractional crystallization models to pressure can provide a general estimate of the size of the original parent body. High-pressure (9 GPa) models were tested in IIAB irons at varying S contents, and it was found that As, Ru, and Rh vs. Au crystallization trends could partially be explained by high pressure. However, crystallization trends for other elements (Ga, Ge, Pd, Sb, Sn, W, Re) are not compatible with high-pressure models. It was concluded in this study that since many of the elements analyzed do not support the case for high-pressure fractional crystallization, this constitutes a failure of the high-pressure model to describe the data. This led to a closer examination of the D(As) vs. S content model at low pressure. A new formalism was developed for a limited range of S contents (18-31 wt.%) pertinent to IIAB evolution. The new formalism for D(As) produces a model consistent with a low-pressure environment with an initial 18 wt.% S content in the IIAB core, and indicates a need for new equations to better describe siderophile element partitioning in magmatic iron meteorites. |
