The first tektite strewn field from Brazil: Geraisites

Autor(en): Gabriel G. Silva, Alvaro P. Crosta, Ludovic Ferrière, Philippe Nonnotte, Eugen Libowitzky, Fred Jourdan
Abstrakt: Tektites are rare natural glasses formed during asteroid impacts on Earth when top layers of soil are melted and launched over great distances, up to thousands of kilometers away from the impact. Although they resemble volcanic glass (such as obsidian), they can be distinguished by features such as extremely low water content[1]. Only seven tektite strewn fields are known worldwide today, including five well-established strewn fields and two that are still in the process of being better described[2,3]. Recently, natural glass specimens were found in northeastern Brazil and a field expedition confirmed their presence across an area at least 90 km long in northern Minas Gerais state[2]. The continued local findings have increased the total number collected to approximately 500 samples, ranging from circa 85g to less than 1 g. The specimens display diverse shapes (such as spherical, ellipsoidal, teardrop, and dumbbell forms) and color ranging from light olive-green to brown, visible in transmitted light in thinner parts. The specimens show a homogeneous chemical composition, contain rare lechatelierite inclusions, and plot in the dacite to rhyolite fields of the total alkali versus silica diagram. FeO contents (1.52 to 3.08 wt.%) are low compared to other known tektites, and water contents, from 70 to 110 ppm, fall within the typical range of known tektites[1,2]. Radiometric dating (⁴⁰Ar/³⁹Ar) indicates that the impact occurred about 6.3 Ma, while Sr/Nd and Lu/Hf isotope data suggest that the impact occurred on continental
crustal rocks, most likely felsic rocks of most Mesoarchean age[2]. No known crater in Brazil has the right set of characteristics (such as age and target rock) to be identified as the source of these tektites. It is a relatively young impact crater, likely formed in granitic terrain, possibly in the São Francisco craton. The tektites were named “geraisites”, after the state of Minas Gerais, where they were first found. It is possible that the geraisites’ strewn field
is much larger than currently estimated and they are scattered on adjacent areas in Brazil.

References
1. Koeberl, C., et al. (2022). Tektites glasses from Belize, Central America: Petrography, geochemistry, and search for a possible meteoritic component. Geochim. Cosmochim. Acta, 325, 232–257.
2. Crósta, A. P., et al. (in press) Geraisite: The first tektite occurrence in Brazil. Geology.
3. Musolino, A., et al. (2025). A new tektite strewn field in Australia ejected from a volcanic arc impact crater 11 Myr ago. Earth Planet. Sci. Lett., 670, 119600.
Organisation(en): Institut für Mineralogie und Kristallographie
Externe Organisation(en): Universidade de São Paulo, Universidade Estadual de Campinas, Natural History Museum Abu Dhabi, Geo-Ocean UMR6538, University of Brest UBO/CNRS/IFREMER/UBS, Institut Universitaire Européen de la Mer IUEM, 29280 Plouzané, France, Curtin University
Publikationsdatum: 12-2025
Peer-reviewed: Ja
ÖFOS 2012: 105116 Mineralogie, 105113 Kristallographie
Link zum Portal: https://ucrisportal.univie.ac.at/de/publications/211b4066-a87f-4592-8824-1ad81bcb5a1b

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