Glass of possible impact origin from Pica (Chile)

Impact glasses are features associated with many terrestrial impact craters or impact structures that can provide important clues for dating impact events or even discovering new impact structures (French & Koeberl, 2010) Also large airbursts have been proposed to be able of melting the soil surface by radiation. As pointed out in French & Koeberl (2010), it is often difficult to ascribe an impact origin to such glasses, which closely resemble non-impact glasses such as fulgurites, volcanic glasses or metallurgical slags. The origin and mechanism of formation of these glasses is thus strongly debated. Hence, proving or disproving the impact origin of these glasses has implications for estimating the flux of airburst producing bolides to the Earth surface. Massive glassy formations (called Pica Glass) were recently discovered in Chile near the town of Pica (Ropert et al., 2017). These glasses have been found in several oucrops forming a line as long as 70 km, lying on the surface along the eastern margin of the Tamarugal– Llamara basin in the Atacama Desert. They have been variously interpreted by several authors as fulgurites, impact glasses, or glasses resulting from large fires melting the surface sediments (See Ropert et al., 2017 and references therein). Also, they have been interpreted as the consequence of a major airburst (Schultz et al., 2021). Preliminary results found by our group strongly suggest the presence of a meteoritic component in the glass, thus indicating that the melt originated from an impact or an airbust.We accurately analysed by FESEM, 15 thin sections made from samples coming from the 4 studied glass outcrops. The samples consist of a vescicular silicate glass (ca 54 wt% SiO2) embedding many clasts of the sand from which the melt probably originated. We found the ubiquitous presence of sulphide blebs of troilite composition . A small fraction of these troilite blebs contains also appreciable amounts of Ni. Other phases of possible meteoritic provenance include: – spherules with a fine intergrowth of Fe phosphide and native iron usually mantled by troilite. – metallic spherules with sizes from 4 up to 20 micrometer and Ni/(Ni+Fe) ratios ranging from 0 to 0.6 (average 0.28 ±0.09). – merrillite and apatite-(Cl). Remarkably, in one sample also two chondrules have been found embedded in the glass which consisted of euhedral diopside crystals, troilite, merrillite, and recristallised mesostasis. Despite other mechanisms may be invoked for the formation of reduced phases, we suggest that the phases we detected are of meteoritic provenance. In particular, despite extensive fires in presence of a reducing agent may be invoked for the formation of metallic Fe spherules, the ancient fire hypothesis (Ropert et al., 2017) would not explain the presence of Fe-Ni alloys. Several issues should be further addressed to better interpret these finds. In our opinion, the mineralogy of the clasts interpreted here as relicts of the projectile, the presence of chondrules, and the presence of numerous Fe-Ni spherules (including two non spherical fragments of kamacite with troilite inclusions) may point to a projectile of ordinary chondritic composition, in contrast to the hypothesis of Schultz et al (Schultz et al., 2021) that instead invokes a cometary projectile.