Aerial crystallization of hematite in impact bombs from the Araguainha astrobleme, Mato Grosso, central Brazil

Optical and scanning electron microscope (SEM) observations of hematite imp act bombs from the Araguainha astrobleme in central Brazil reveal an aerody namically stable microstructure, which is interpreted to reflect the aerial crystallization of hypervelocity melt globules derived from Fe-rich sedime ntary target rocks. The bombs are composed of an aggregate of preferentiall y oriented conic units with different degrees of packing, giving rise to po rous and massive hematite varieties. These varieties generally occupy the f ront and rear of the bombs, respectively. Quartz fragments with planar defo rmation features are common in the porous domains and represent ejected sol id particles captured by the bombs during cooling. The absence of typical c ontorted shapes in these bombs reflects the lack of spin during their balli stic trajectory. The bombs are enriched in Rh (4x); Ru, Ir, and Pd (5x); an d Pt (>30x) relative to the indigenous level of target sediments. This sugg ests the presence of a projectile-derived component in their constitution. Single composition oxidic bombs (pure hematite and pure silica) predominate in this occurrence, indicating that a mechanism of segregation has operate d in the ejected melt volumes.