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Hematite impact “bombs” are one of the most striking (and enigmatic) features of the large Araguainha impact structure in central Brazil. They have both porous or massive textures, elongated shapes from 5 to 50 cm in diameter, and botryoidal textures that suggest hydrothermal origin. Some authors have considered these objects as a possible analog of hematite nodules found in Mars, and consequently related to a hydrothermal system. Here we report rock magnetic measurements, X-ray diffraction and Mössbauer spectra for both massive and porous samples for a detailed description of the hematite. Room temperature magnetic measurements, including hysteresis loops, back-field and saturation magnetization acquisition, FORC, as well as X-ray diffraction and Mössbauer experiments are compatible with both massive and porous types being almost pure hematite. Room temperature FORCs after heating in a He atmosphere show two peaks; the original high-coercivity peak of hematite and a low-coercivity one (probably maghemite) at the Bc and Bb origin, thus indicating significant modification of the magnetic mineralogy of the material during thermal treatment in reducing conditions. However, conditioning in an oxidizing environment (heating in air) seemed to block generation of this low coercivity material in subsequent heating in a reducing (Ar) atmosphere. Therefore, we conclude that this material was not heated greatly in its generation. This would not be likely for impact-ejected bombs, so origin from post-impact hydrothermal activity seems likely.

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Geochemistry Geophysics Geosystems





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© 2011 American Geophysical Union. View original article in Geochemistry Geophysics Geosystems, DOI: 10.1029/2011GC003758.





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