Distinguishing volcanic from impact glasses—The case of the Cali glass (Colombia)

Abstract Natural glass occurs on Earth in different geological contexts, mainly as volcanic glass, fulgurites, and impact glass. All these types of glasses are predominantly composed silica with variable amounts impurities, especially the alkalis, differ their water content due to mode formation. Distinguishing between glasses, also Moon other planetary bodies, can be challenging. This is particularly true for origin. Because often used determination age events, even if out context, well derive pressure temperature constraints, or evaluate volatile contents magmas source regions, we rely methods that unambiguously distinguish glasses. We case Cali found an extended area close city western Colombia, which was previously suggested origin, show that, using a multimethod approach (i.e., combining macroscopic observations, chemical isotopic data, H2O content), it possible formation modes. A suite samples analyzed electron microprobe, instrumental neutron activation analysis, thermal ionization mass spectrometry, Fourier-transform infrared spectroscopy, allowing us definitively exclude origin instead classify rhyolitic (obsidian). Our results suggest “unusual occurrences” claimed, but not convincingly proven, should reexamined same methodology applied here.