The response of zircon to the extreme pressures and temperatures of a lightning strike.

Hypervelocity impacts can produce features in zircon that are not normally produced by endogenic processes. However, lightning can also induce extreme pressure-temperature excursions, and its effect on zircon has not been studied. With the aim to recognise features that form in response to extreme pressure-temperature excursions but are not unique to hypervelocity impacts, we imaged and undertook microstructural characterization of zircon in a fulgurite (a tubular body of glass and fused clasts that formed in response to a lightning strike). We document zircon with granular ZrO and rims of vermicular ZrO, features which vary in abundance with increasing distance from the fulgurite's central void. This indicates that these features formed in response to the lightning strike. Zircon dissociation to ZrO and SiO is a high-temperature, relatively low-pressure phenomenon, consistent with previous suggestions that lightning strikes involve extreme temperatures as well as pressures greater than those usually generated in Earth's crust but rarely > 10 GPa. The rims of monoclinic ZrO record crystallographic evidence for precursor cubic ZrO, demonstrating that cubic ZrO is not unique to hypervelocity impacts. Given the likelihood that this fulgurite experienced pressures of, at most, a few GPa, evidence for cubic ZrO indicates peak temperatures > 2000 °C.