Abstract
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 ZrO2 and rims of vermicular ZrO2, 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 ZrO2 and SiO2 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 ZrO2 record crystallographic evidence for precursor cubic ZrO2, demonstrating that cubic ZrO2 is not unique to hypervelocity impacts. Given the likelihood that this fulgurite experienced pressures of, at most, a few GPa, evidence for cubic ZrO2 indicates peak temperatures > 2000 °C.
Highlights
Hypervelocity impacts can produce features in zircon that are not normally produced by endogenic processes
Granular ZrO2 and rims of vermicular Z rO2 occur in zircon in the York County fulgurite but not the control sample. The abundance of these features varies systemically with increasing distance from the fulgurite’s central void. We consider this unequivocal evidence that granular and vermicular ZrO2 in zircon formed in response to the lightning strike and we use these features to constrain pressure–temperature conditions reached in the fulgurite
It has been suggested that fulgurites can experience pressures of a few GPa, perhaps occasionally exceeding 10 GPa23,24, and we cannot assume that zircon grains in fulgurites only experienced atmospheric pressures
Summary
Hypervelocity impacts can produce features in zircon that are not normally produced by endogenic processes. Minor occurrences of ZrO2 in granular zircon (e.g., refs.8,12,13) is indicative of zircon dissociation, experimentally constrained to occur above 1673 °C at 1 atm[2,10,11], whereas the formation of rims of vermicular zirconia around zircon grains can indicate temperatures in excess of 2370 °C14,15 Confirmation of the latter requires reconstruction of the polymorphic transformation history of this zirconia rim from microstructural data (so-called ‘phase heritage’14); a rim of monoclinic zirconia (baddeleyite) can preserve crystallographic relationships (namely, spatially-clustered baddeleyite grains with distinctive patterns of up to twelve orientations) that uniquely identify the former presence of a precursor cubic zirconia p olymorph[14,15]
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