Zircon microstructures record high temperature and pressure conditions during impact melt evolution at the West Clearwater Lake impact structure, Canada

Abstract

Evolution of impact melt in terms of initial melt temperatures, melt transport, and cooling history, is a process that remains to be fully understood. Theoretical predictions had suggested that impact melts can experience temperatures far exceeding those in endogenous igneous settings. Direct evidence of the hottest temperatures recorded in impactites was observed recently at the Mistastin Lake impact structure, Canada. The former presence of cubic zirconia, a polymorph of ZrO2 that forms at >2370 °C, was documented within impact glass. In this work, we investigated the zircon and zirconia microstructures and crystallographic orientation relationships with electron backscatter diffraction in two impact glass samples from West Clearwater Lake impact structure in Quebec, Canada. Here we present the first report of the former presence of cubic zirconia, indicating a superheated melt temperature of >2370 °C in one of two impact glass samples analysed. Our results make West Clearwater Lake impact structure the second terrestrial structure with confirmed evidence of former cubic zirconia. Furthermore, we found evidence of melt superheating to temperature of 1673 °C in the other impact glass sample. We also document the first occurrence of former reidite in granular neoblastic (FRIGN) zircon grains in the two impact glass samples analysed in this work, giving us a minimum shock pressure estimate of 20 GPa. This study highlights the heterogeneous thermodynamic (high temperature/low pressure, high pressure, and low temperature/ low pressure) conditions recorded within impact glass from West Clearwater Lake impact structure.