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. This indicates that these features formed in response to the 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.
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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). However, lightning can also induce extreme pressure-temperature excursions, and its effect on zircon has not been studied. Hypervelocity impacts can produce features in zircon that are not normally produced by endogenic processes. When the peak current is greater, the wave front time is shorter, the half-value time is longer, the resistivity is greater, or the specific heat is more, the overall temperature in the sandstone is greater, and the phase change region is larger under the same conditions. Within the sandstone, both the temperature isosurface and phase transition area show a hemispherical distribution. The temperature varies most dramatically around the lightning strike point, and the area affected by lightning strikes decreases as the horizontal plane deepens. The entire dynamic process exhibits a logarithmic growth tendency, with rapid development of temperature and phase change areas during the early stages of lightning current, followed by gradual increases until constant. The results indicate that a lightning strike generates a large amount of electromagnetic heat in the sandstone, causing it to heat up and melt or even vaporize.
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The transient calculation is performed to investigate the dynamic change process within the sandstone during lightning strikes, and the effects of related factors such as the peak current of the lightning current, waveform parameters, and electrothermal parameters of the sandstone on the model results are analysed. The model is validated by comparing field observations and theoretical analyses from the previously published studies. This model considers the spatial Gaussian distribution of lightning current. In order to examine the thermal effect of lightning currents on sandstone, an electro-thermal coupling model based on electromagnetic field and heat transfer theory was constructed.