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

Ludovic Ferrière,Fabio Iwashita,Eugen Libowitzky,Wencke Wegner,Alvaro P Crósta,Christian Koeberl

doi:10.1130/g48925.1

Abstract

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

Highlights

Natural glass is rare on Earth compared to crystalline rocks due to its specific formation conditions and durability aspects
The distinction between volcanic glass versus impact glass can be challenging and may lead to erroneous interpretation of the geological context (French and Koeberl, 2010); the potential for misidentification of origin motivated our investigation of Cali glass
By combining a number of different analytical methods and following a relatively simple research methodological scheme, we can discriminate between a volcanic origin and an impact origin for the Cali glass

Summary

Introduction

Natural glass is rare on Earth compared to crystalline rocks due to its specific formation conditions and durability aspects (i.e., glass is metastable and altered). Cali in western Colombia) samples to unravel the origin of this “unusual” glass (Fig. 1).

Results

Conclusion