Geochemistry

Carrying Iridium Around the World

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Science  02 Mar 2001:
Vol. 291, Issue 5509, pp. 1665
DOI: 10.1126/science.291.5509.1665b

The Cretaceous-Tertiary (K-T) impact event occurred about 65 million years ago and is considered the main cause for the extinction of dinosaurs and other species at that time. Evidence for this impact includes the large impact crater detected at Chicxulub on the coast of the Yucatan Peninsula of Mexico and a globally distributed thin layer of clay minerals at the K-T boundary that contains shocked quartz and is relatively enriched in iridium.

Wdowiak et al. performed a series of cryogenic Mössbauer spectroscopic measurements on samples from the uppermost section of the boundary layer collected from sites in Central and North America and in Europe. These upper sections are distinctly redder than the lower sections and enriched in hydrous iron oxides or iron sulfates. The spectroscopic data identifies nanometer-size particles (nanoparticles) of goethite (α-FeOOH) in most of the samples while the sections sampled in Italy are enriched in hematite (α-Fe2O3). These nanoparticles may represent the vaporized remnants of the impactor. Modeling suggests that most of the impactor probably was vaporized, and small particles left over from this ejection and vaporization may have settled out of atmosphere at a later time, which would help populate the globally distributed boundary layer. Thus, these nanoparticles may provide clues to the composition of the impactor, which is unknown, and information about the dynamics of the event. These nanoparticles also may be the carrier phase for the iridium, a siderophile element, whose microstructural location also is unknown. — LR

Meteorit. Planet. Sci.36, 123 (2001).

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