EVIDENCE FOR DYNAMIC MELTING, EMPLACEMENT, AND CARBONACEOUS IMPACTOR ENTRAINMENT IN PICA GLASS FROM THE CHILEAN ATACAMA DESERT

Musolino et al. (METSOC, 2024) reported the ability to produce baddeleyite corona on zircons by flux melting for 6 hours between 900 and 1000⁰C. They suggest their results are sufficient to discount the conclusion that decomposed zircons in Pica glass (PG) formed by exposure to ultra-high temperatures in an air burst (Schultz et al., Geology, 2022). But the observation that such corona can be produced at low temperatures by smelting is not new; they occur in archeological glass. While those instances and these experiments demonstrate that limited zircon decomposition can occur with sustained temperatures >900⁰C in a concentrated flux, these are not the conditions expected from combustion of a thin biomass layer, i.e., their counterhypothesis for PG. Furthermore, they choose to ignore documented zircons in PG and other impact deposits that exhibit complete decomposition and are quenched with residual silica.

In a companion abstract, those authors note that decomposed and non-decomposed zircons in PG occur nearby in the same thin section. In a static melting environment (as they propose), adjacent zircons would be expected to experience similar thermochemical conditions and styles of decomposition. Contrarily, PG samples clearly indicate that they were formed in a highly dynamic environment.

Patterns of heterogeneous decomposition and melting demonstrate that many PGs formed by convective mixing of material, consistent with commonly twisted and aerodynamic forms. Some locations show possible evidence of ejected fine material from a central melt zone. We also observe cases where indurated sediments are melted along sheared laminations. Static heating should have melted the interstitial sediments uniformly.

Samples from most all PG locations demonstrate contamination by meteoritic materials, consistent with a carbonaceous body bearing assemblages like those found in comets or aqueously altered asteroids. An exception is a location reported by Roperch at al. (EPSL, 2017) that can be shown to be slag associated with an archeological site. The common character of entrained exogeneous clasts over large distances precludes inheritance from pre-event sediments. If melting were static, some surviving phases in these clasts would have melted (especially if a flux was important). Instead, these clasts were entrained into a melt already forming and in the process of quenching.