Melting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure, Brazil

摘要

Abstract–Shatter cones are one of the most widely recognized pieces of evidence forudmeteorite impact events on Earth, but the process responsible for their formation is stilluddebated. Evidence of melting on shatter cone surfaces has been rarely reported in theudliterature from terrestrial impact craters but has been recently observed in impactudexperiments. Although several models for shatter cones formation have been proposed, soudfar, no one can explain all the observed features. Shatter cones’ from the Vista Alegreudimpact structure, Brazil, formed in fine-grained basalt of the Jurassic-Cretaceous SerraudGeral Formation (Parana large igneous province). A continuous quenched melt film,udconsisting of a crystalline phase, mica, and amorphous material, decorates the striatedudsurface. Ultracataclasites, containing subrounded pyroxene clasts in an ultrafine-grainedudmatrix, occur subparallel to the striated surface. Several techniques were applied toudcharacterize the crystalline phase in the melt, including Raman spectroscopy andudtransmission electron microscopy. Results are not consistent with any known mineral, butudthey do suggest a possible rare or new type of clinopyroxene. This peculiar evidence ofudmelting and cataclasis in relation with shatter cone surfaces is interpreted as the result ofudtensile fracturing at the tip of a fast propagating shock-induced rupture, which led to theudformation of shatter cones at the tail of the shock front, likely during the early stage of theudimpact events.