Fe-carbide and Fe-sulfide liquid immiscibility in IAB meteorite, Campo del Cielo: Implications for iron meteorite chemistry and planetesimal core compositions

摘要

The majority of iron meteorites evolved in a relatively low-sulfur environment whereas chondritic meteorites tend to be comparatively sulfur-rich. Since liquid iron should incorporate sulfide minerals as it migrates from its chondritic source, this represents something of a conundrum in our understanding of iron meteorite formation, and by association, of asteroid core formation. Here, we investigate a series of samples of the Campo del Cielo suite of silicate-bearing (nonmagmatic) iron meteorites, which come from a single fall event. These likely formed in an impact-related process by rapid accumulation of liquid metal and incorporation of silicate clasts. We model the competing processes of rapid crystallisation of metal and flotational separation of the silicate clasts to provide a basis for understanding the fractionation of the Fe-Ni-S-C-P-(Cr-O) liquid. A combination of textural analysis of complex metal-graphite and sulfide veins and networks, laser ablation analysis of metal and sulfides, and published phase relations, are used to show that fractionation promoted evolution to a system with immiscible Fe-carbide and Fe-sulfide liquids. We suggest that this development of immiscibility allowed the silicate clasts to become enriched in S, C and P as they underwent flotational separation. This process left a large accumulation of light element-depleted FeNi metal, represented by the bulk of the Campo del Cielo meteorites. If similar large metal accumulations were able to form through impacts during the growth stage of planetesimals, preferential settling of these upon subsequent silicate partial melting would promote formation of light element-depleted cores. Furthermore, because the silicate clasts controlled fractionation of the metallic liquid in Campo del Cielo, we suggest that; (1) trace element trends within the silicate-bearing iron meteorite groups reflect the duration of interaction between liquid metal and clasts, and (2) that the differences in trace element chemistry between fractionally crystallised (magmatic) and silicate-bearing iron meteorites are best explained by a combination of the presence or absence of these clasts, and the duration of metal crystallisation.