Decomposed zircons: A reliable geothermometer to identify impact and airburst glasses?

When exposed to high-temperature conditions (~1670°C), zircon crystals (ZrSiO4) decompose according to the reaction: ZrSiO4→ZrO2+SiO2 [2,8]. Under optical and electron microscopes, decomposed zircons are easily identified by the presence of bright rims of baddeleyite (ZrO2) surrounding the unaltered primary crystal core (ZrSiO4). Due to the high temperatures needed for this reaction to occur (i.e., exceeding the highest temperatures normally reached by magmatic processes or wildfires on the Earth’s surface), finding decomposed zircons in natural glass has become a handy unequivocal way to relate natural glass to extreme processes like meteoritical impacts [1], airbursts [6], or lightning [3]. If recognizing fulgurites (i.e., products of lightning) is more easily done because of their morphology, the identification of impact glasses can be problematic, especially when they are not associated with a known impact crater. This work aims to demonstrate the reliability of zircon decomposition as a geothermometer, used to identify impact (or airbursts) glasses. Through high-temperature experiments, we show that the decomposition of zircons can occur in natural systems at lower temperatures than the ones predicted by models. At T=900-1000°C (P=1 bar, exposed to air), in the presence of Ca-sulfates and NaCl-rich soil called ‘caliche’ (from the Atacama Desert, chosen for its relation with one of the most recent debated case, that of Pica glass – [4,5,6,7]), zircons decomposed forming the typical bright rims. Using FEG-SEM-EDS, Raman spectroscopy, and TEM (on thin foils prepared using FIB), however, we show that the Zr-rich rim mineralogy in our experiments differs from previous petrographic descriptions, with assemblages of baddeleyite, baddeleyite + Ca-Zr-oxide, or only Ca-Zr-oxide. In conclusion, we demonstrate that decomposed zircons could also result from lower temperature processes than impacts or airbursts and should be used more carefully in assessing the origin of glasses. Also, we suggest that a more detailed mineralogical characterization of decomposed zircons (rarely done after their detection) is needed to correctly assess the formation conditions of samples containing such rims.   References [1] El Goresy A., 1965. Baddeleyite and its significance in impact glasses. Journal of Geophysical Research, 70:3453-3456. [2] Kaiser A., et al., 2008. Thermal stability of zircon (ZrSiO4). Journal of the European Ceramic Society, 28:2199-2211. [3] Kenny G.G. and Pasek M.A., 2021. The response of zircon to the extreme pressures and temperatures of a lightning strike. Scientific Reports, 11:1560. [4] Roperch P., et al., 2017. Surface vitrification caused by natural fires in Late Pleistocene wetlands of the Atacama Desert. Earth and Planetary Science Letters, 469:15-26. [5] Roperch P., et al., 2022. Widespread glasses generated by cometary fireballs during the late Pleistocene in the Atacama Desert, Chile: COMMENT. Geology, 50.5:e550. [6] Schultz P.H., et al., 2022. Widespread glasses generated by cometary fireballs during the late Pleistocene in the Atacama Desert, Chile. Geology, 50.2:205-209. [7] Schultz P.H., et al., 2022. Widespread glasses generated by cometary fireballs during the late Pleistocene in the Atacama Desert, Chile: REPLY. Geology, 50:e551. [8] Timms N.E., et al., 2017. A pressure-temperature phase diagram for zircon at extreme conditions. Earth-Science Reviews, 165:185-202.