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Title: Geochronological constraints on the age of a Permo-Triassic impact event: U-Pb and 40Ar/39Ar results for the 40 km Araguainha structure of central Brazil
Authors: E. Tohver, C. Lana, P.A. Cawood, I.R. Fletcher, F. Jourdan, S. Sherlock, B. Rasmussen, R.I.F. Trindade, E. Yokoyama, C.R. Souza Filho, Y. Marangon

Impact cratering has been a fundamental geological process in Earth history with major ramifications for the biosphere. The complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination, hampering the assessment of the effects of an individual event in the geological record. We demonstrate the utility of a multi-chronometer approach in our study of samples from the 40 km diameter Araguainha impact structure of central Brazil. Samples of uplifted basement granite display abundant evidence of shock deformation, but U/Pb ages of shocked zircons and the 40Ar/39Ar ages of feldspar from the granite largely preserve the igneous crystallisation and cooling history. Mixed results are obtained from in situ40Ar/39Ar spot analyses of shocked igneous biotites in the granite, with deformation along kink-bands resulting in highly localised, partial resetting in these grains. Likewise, spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material, the age of the glass itself, and post-impact devitrification. The timing of crater formation is better assessed using samples of impact-generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization. Granular aggregates of neocrystallised zircon form a cluster of ten U-Pb ages that yield a "Concordia" age of 247.8 ± 3.8 Ma. The possibility of Pb loss from this population suggests that this is a minimum age for the impact event. The best evidence for the age of the impact comes from the U-Th-Pb dating of neocrystallised monazite and 40Ar/39Ar step heating of three separate populations of post-impact, inclusion-rich quartz grains that are derived from the infill of miarolitic cavities. The 206Pb/238U age of 254.5 ± 3.2 Ma (2sigma error) and 208Pb/232Th age of 255.2 ± 4.8 Ma (2sigma error) of monazite, together with the inverse, 18 point isochron age of 254 ± 10 Ma (MSWD = 0.52) for the inclusion-rich quartz grains yield a weighted mean age of 254.7 ± 2.5 Ma (0.99%, 2sigma error) for the impact event. The age of the Araguainha crater overlaps with the timing of the Permo-Triassic boundary, within error, but the calculated energy released by the Araguainha impact is insufficient to be a direct cause of the global mass extinction. However, the regional effects of the Araguainha impact event in the Paraná-Karoo Basin may have been substantial.

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Title: Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structure
Authors: C.H. Tong et al., Dept. of Earth and Planetary Sciences, Birkbeck, Univ. of London, Malet Street, London WC1E 7HX, UK.

Tong et al. present new geophysical results showing the internal structure of the largest impact crater in South America (Araguainha in Brazil). The formation of impact craters is a dramatic, high-energy geological event and involves rapid ejection and movement of Earth materials. Computer models help to reconstruct these impact events, but how the crustal materials settled to result in the final form of the craters has been poorly supported by geological evidence. The research of Tong et al. shows, for the first time, the geological evidence for the resurgence of Earth materials to the center of an impact crater after the initial ejection. Their observation is made possible by the novel application of geophysical methods involving pumping electricity to the ground for detecting the shape and location of the different rock types inside the impact crater. They conclude that the form of an impact crater is determined by the pre-impact rock formations. This conclusion has major implications in the understanding of the interior of planetary bodies. Tong et al.'s results suggest that the form of impact craters on different rocky planetary bodies is a surface manifestation of their internal structures.

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The Araguainha Crater or Araguainha Dome is an impact crater on the border of Mato Grosso and Goiás states, Brazil, between the villages of Araguainha and Ponte Branca. With a diameter of 40 km, it is the largest known impact crater in South America, and possibly the oldest one. The crater was formed 244.40 ± 3.25 million years ago in the Triassic era, when the region was probably a shallow sea.
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Latitude:  16°48'46.43"S, Longitude:  52°59'23.65"W

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