A 650 km2 Miocene strewnfield of splash-form impact glasses in the Atacama Desert, Chile

Glassy ejecta are associated to a limited number of impact craters, and yet hold key information about hypervelocity impact processes. Here we report on the discovery of a ~650 km2 impact glass strewnfield in the Central Depression of the Atacama Desert. These cm-sized splash-form objects, that we refer to as atacamaites, are essentially composed of a dacitic glass formed by high-temperature melting of local magmatic rocks, with the addition of a variable iron meteorite contamination, 5 wt.% on average. The most likely nature for the impactor is the IIAB iron group. The fission-track plateau method, on two samples, yielded a mean formation age of 7.83 ± 0.26 Ma. No associated impact crater has been discovered so far, suggesting it may be a relatively small, km-sized crater. The glassy nature, aerodynamic shapes, elevated formation temperature, and low water content are reminiscent of tektites. However, their small size, heterogeneity, oxidation state, significant contamination by the impactor, and likely more proximal provenance distinguish them from tektites. Atacamaites have no equivalent among the few known terrestrial ejected impact glasses, and increase the intriguing diversity of such products that we propose to name "tektoids".