A two stage impact melting process in an impact glass strewn field from the Atacama Desert

A new type of silica-rich glass has been discovered associated with the known impact glass strewn field of the Atacama Desert. Based on petrography, chemical composition and indistinguishable ⁴⁰Ar/³⁹Ar formation ages at circa 6.6 Ma, we infer that these two glasses were produced by the same impact event, which gave rise to two successive compositionally different melt batches in close succession. The first one is a silica-rich melt derived from a mixture of quartz sand and weathered magmatic rocks. It is reduced and devoid of extraterrestrial contamination. The second one, much more abundant and which corresponds to the normal glass, is oxidised, highly contaminated by the iron type impactor and derived from an underlying unweathered dacitic rock. This scheme sheds a new light on the first second of the interaction between the Earth surface and a large metallic asteroid.