Historic Discovery in Brazil
Scientists have announced the discovery of Brazil's first tektite field, a significant geological find that provides compelling evidence of a massive asteroid impact approximately 6.3 million years ago. These natural glass fragments, officially named 'geraisites' after the state of Minas Gerais where they were initially found, stretch across an expansive area of over 900 kilometers.
The groundbreaking research was led by Professor Álvaro Penteado Crósta, a geologist at the Institute of Geosciences at the University of Campinas (IG-UNICAMP), in collaboration with an international team of researchers from Brazil, Europe, the Middle East, and Australia. The findings were published in the prestigious journal 'Geology'.
This discovery places Brazil among a select group of nations hosting major tektite fields, joining the ranks of Australasia, Central Europe, the Ivory Coast, North America, and Belize.
Characteristics of the Geraisites
Tektites are unique natural glasses formed when terrestrial rocks are melted by the extreme heat and pressure of a meteorite impact, then ejected into the atmosphere, cooling rapidly into glass fragments. Over 600 samples of geraisites have been collected to date.
These fragments vary in size, ranging from less than 1 gram to 85.4 grams and reaching up to approximately 5 centimeters in length. They exhibit diverse aerodynamic shapes, including:
- Spherical
- Ellipsoidal
- Drop-shaped
- Discoid
- Dumbbell-shaped
- Twisted forms
A crucial characteristic confirming their tektite classification is their exceptionally low water content, measured between 71 and 107 parts per million. This contrasts sharply with volcanic glass like obsidian, which typically contains at least 700 parts per million of water. Geochemical analyses also revealed high levels of silicon dioxide (70.3% to 73.7%) and a combined sodium oxide and potassium oxide content of 5.86% to 8.01%. Isotopic geochemistry suggests the molten material originated from ancient continental crust, approximately 3 to 3.3 billion years old, likely pointing to the São Francisco craton.
Extent and Age of the Impact Event
The initial discovery of geraisites occurred in three municipalities in northern Minas Gerais: Taiobeiras, Curral de Dentro, and São João do Paraíso, covering an area of about 90 kilometers. Subsequent discoveries in the neighboring states of Bahia and Piauí have expanded the known distribution of the tektite field to more than 900 kilometers.
Dating based on argon isotope ratios indicates that the impact event took place approximately 6.3 million years ago, coinciding with the end of the Miocene epoch. Researchers note that this age should be interpreted as a maximum, as some argon may have been inherited from the ancient rocks targeted by the impact.
Significance for South American Geology
While no associated impact crater has yet been identified, this is not uncommon for tektite fields, as only three of the six major fields worldwide have known craters. The sheer volume of molten material and the vast dispersion area strongly suggest a significant impact event, with researchers concluding that the impacting body was unlikely to have been small.
This discovery is crucial for filling a notable gap in South America's impact record, where few large impact structures are known, and most are considerably older. The finding also reinforces the idea that tektites may be more common globally than previously believed.
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