Geraisite: The first tektite occurrence in Brazil

Abstract Tektites, a rare type of impact glass, are known to occur only in five distinct strewn fields on Earth. We report on the discovery of a new tektite strewn field in northeastern Brazil. This recent finding resulted in the collection of ~500 specimens found within a strewn field at least 90 km long. Mostly black in color, the masses, hereby named “geraisites” after the Brazilian state of Minas Gerais, range from <1 g up to 85.4 g and exhibit various shapes, rarely showing lechatelierite inclusions. In terms of chemical composition, they fall in the dacite and rhyolite fields of the total alkali versus silica diagram, with similar SiO2 and slightly higher Na2O + K2O content in comparison with other known tektites. Some variations in trace elements from sample to sample, such as for Cr and Ni, are observed. Water contents of the glass are extremely low, ranging between 70 and 110 ppm. The best estimate for their age of formation measured by the 40Ar/39Ar method is currently ca. 6.3 Ma, although the possible presence of inherited 40Ar* will require more analyses to fully establish their age. Strontium, Nd, and Hf isotopic compositions as well as Nd and Hf model ages provide insights into the age and composition of the source rocks, having a continental crustal composition, most likely Mesoarchean felsic rocks.