Melting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure,Brazil |
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Authors: | Lidia Pittarello Fabrizio Nestola Cecilia Viti Alvaro Penteado Crósta Christian Koeberl |
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Affiliation: | 1. Department of Lithospheric Research, University of Vienna, Vienna, Austria;2. Department of Geosciences, University of Padova, Padova, Italy;3. Department of Physics, Earth and Environmental Sciences, University of Siena, Siena, Italy;4. Institute of Geosciences, Campinas SP, Brazil;5. Natural History Museum, Vienna, Austria |
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Abstract: | Shatter cones are one of the most widely recognized pieces of evidence for meteorite impact events on Earth, but the process responsible for their formation is still debated. Evidence of melting on shatter cone surfaces has been rarely reported in the literature from terrestrial impact craters but has been recently observed in impact experiments. Although several models for shatter cones formation have been proposed, so far, no one can explain all the observed features. Shatter cones' from the Vista Alegre impact structure, Brazil, formed in fine‐grained basalt of the Jurassic‐Cretaceous Serra Geral Formation (Paraná large igneous province). A continuous quenched melt film, consisting of a crystalline phase, mica, and amorphous material, decorates the striated surface. Ultracataclasites, containing subrounded pyroxene clasts in an ultrafine‐grained matrix, occur subparallel to the striated surface. Several techniques were applied to characterize the crystalline phase in the melt, including Raman spectroscopy and transmission electron microscopy. Results are not consistent with any known mineral, but they do suggest a possible rare or new type of clinopyroxene. This peculiar evidence of melting and cataclasis in relation with shatter cone surfaces is interpreted as the result of tensile fracturing at the tip of a fast propagating shock‐induced rupture, which led to the formation of shatter cones at the tail of the shock front, likely during the early stage of the impact events. |
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