Iron cosmic spheres, composed of iron oxides surrounding a FeNi metal core, were discovered more than a century ago during the voyage of the HMS Challenger1. That tbe spheres originated by atmospheric melting of meteoritic metal is supported by a variety of evidence, including elemental composition2, the presence of the metastable oxide wustite3, similarity to analogues produced in the laboratory4 and content of 53Mn produced by exposure to cosmic rays5. We report here a previously unrecognized property of the spheres. The most common spheres larger than 300 µm do not, in fact, contain FeNi metal cores, but instead contain a micrometre-sized nugget composed almost entirely of platinum group elements. These elements appear to have been concentrated by the oxidation of molten meteoritic metal during atmospheric entry. This process is critically dependent on the relative abundance of oxygen in the atmosphere, and the first appearance of the nuggets in the geological record may provide a marker indicating when the oxygen abundance attained half of its present level.
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Brownlee, D., Bates, B. & Wheelock, M. Extraterrestrial platinum group nuggets in deep-sea sediments. Nature 309, 693–695 (1984). https://doi.org/10.1038/309693a0
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