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

Abstract— Optical and scanning electron microscope (SEM) observations of hematite impact bombs from the Araguainha astrobleme in central Brazil reveal an aerodynamically stable microstructure, which is interpreted to reflect the aerial crystallization of hypervelocity melt globules derived from Fe-rich sedimentary target rocks. The bombs are composed of an aggregate of preferentially oriented conic units with different degrees of packing, giving rise to porous and massive hematite varieties. These varieties generally occupy the front and rear of the bombs, respectively. Quartz fragments with planar deformation features are common in the porous domains and represent ejected solid particles captured by the bombs during cooling. The absence of typical contorted shapes in these bombs reflects the lack of spin during their ballistic trajectory. The bombs are enriched in Rh (4×); Ru, Ir, and Pd (5×); and Pt (>30×) relative to the indigenous level of target sediments. This suggests 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 operated in the ejected melt volumes.