About 190 meteorite impact craters are known on Earth, but only five of them formed tektite fields. This therefore shows that the discovery of a sixth vast deposit of this dark natural glass, specially forged by the impact of a meteorite, is an event. This study carried out in Brazil, published on Geology and relayed by Gizmodo, helps to unravel the almost unexplored history of ancient impacts in South America. However, scientists continue to search for the location of the crater.
The discovered tektite dispersion field, which extends over 900 kilometers, finds its origin in the probably spectacular impact of a meteorite, 6.3 million years ago. An international team of researchers collected around 500 fragments of these brand new tektite specimens. They are now called geraisites, in reference to the Brazilian state in which they were discovered: Minas Gerais, in the southeast of the country.
A rare and complex training
If this discovery is a landmark, it is because the formation of tektite is as rare as it is complex. It takes a meteorite to hit the Earth, then the Earth’s rocks to melt and be ejected into the atmosphere. When they pass through the atmosphere, the molten drops take different shapes: spheres, peanuts, dumbbells…
The tektites then cool in the atmosphere and fall back to Earth, forming dispersion fields. The best known of these is Australasia, straddling Asia and Oceania, which covers between 10 and 30% of the earth’s surface according to estimates. Other tektite fields are found in Central Europe, Ivory Coast, North America and Belize.
These were the first finds in the towns of Taiobeiras, Curral de Dentro and São João do Paraíso, in the north of the state of Minas Gerais, which put the geologists behind this discovery in Brazil on the trail. This first zone was then only 90 kilometers long. After a first published study, other tektites were found in Bahia and Piauí. The size of the field has thus increased tenfold.
“The expansion of the search area corresponds to what we observe when other dispersal fields are discovered around the world”observes Álvaro Crósta, lead author of the study and professor of geology at the State University of Campinas. “The size of the field depends on the energy at the impact of the meteorite.”
A crater still not found
While they have evidence that these glass fragments originated from a meteorite crashing into Earth, scientists are still searching for the impact crater. But this is not unusual: this is in fact the case for half of the known tektite fields. Here, researchers believe that the Geraisites Crater has a good chance of being in the São Francisco Craton, a geological region located in the eastern part of the South American continental crust. Future studies will try to confirm this.
In the meantime, the study of tektite fragments is in full swing. These weigh between 1 and 85.4 grams and can reach a wingspan of up to 5 centimeters. Although they appear black at first glance, they actually take on a very distinctive gray-green hue and become translucent under strong light.
“One of the main criteria for qualifying geraisites as tektites was their very low water content. Infrared spectroscopy revealed a rate between 0.0071% and 0.0107%explains geologist Álvaro Crósta. For comparison, volcanic glasses like obsidian typically contain between 0.07% and 2% water. Tektites are known to be much drier.”
Geraisites are mainly composed of silica. They contain slightly higher concentrations of sodium and potassium oxides than other types of tektites. The researchers also measured the content of lechatelierite, a vitreous silica that forms in extreme heat. Researchers dated the geraisites to a meteorite impact that occurred 6.3 million years ago. The world was then approaching the end of the Miocene epoch, a period which extends from approximately 23 to 5 million years ago, and which marks the divergence of the first hominids from chimpanzees.
