Low-temperature dissociation of zircon in highly alkaline conditions: A cautionary note for studies on natural glasses of debated origin

Dissociated zircon is largely used as a robust indicator of glasses generated by impact cratering and airbursts. The reaction of zircon dissociation, i.e. ‘ZrSiO₄ → ZrO₂ + SiO₂’, requires high temperatures (>1670°C) only reached by extreme geological processes. Using high-temperature experiments, this study shows that zircon can dissociate and form ZrO₂-rich coronitic rims at temperatures of 900–1000°C (P = 1 bar), in the presence of a specific chemical environment made of NaCl or a mixture of NaCl and caliche soil (Ca-sulfates). The use of silica glass vessels provides a SiO₂-rich environment during the experiments. We observe that the dissociation is strongly related to the complexity of the surrounding system (e.g. the presence of other minerals that act as a flux) in which the reaction occurs. For these reasons, we suggest considering a more careful approach in using dissociated zircon as indicative of very high temperatures in glass-forming processes.