Oxidation state of iron in fulgurites and Trinitite: Implications for redox changes during abrupt high-temperature and pressure events

Abstract

Understanding the geochemical effects of shock metamorphism that occur on planetary surfaces is critical when using potentially shocked planetary samples. Fulgurites, formed by lightning strikes on Earth’s surface, could provide an analog for understanding shock metamorphism, because the intense heat and pressure of impact events are similar to those experienced during lightning strikes. The oxidation state generated during impact is the result of temperature (above 2000 K) and pressure (above 10 GPa) conditions, though the composition and physical nature of the country rock are also important. Atomic explosions such as the Trinity nuclear test and the resulting melted surface material Trinitite can also serve as proxies of shock metamorphism. This study compares samples of fulgurite and Trinitite glasses with the associated country rock using petrography, backscattered electron images, and Mössbauer spectroscopy, with a focus on evaluating redox changes associated with lightning-induced and atomic explosion metamorphism as a function of target material. Two fulgurite melts were also subjected to additional analysis using X-ray absorption spectroscopy to assess potential spatial variations in redox state via in-situ microscale measurements. Results indicate lightning-induced redox variations are heterogeneous at micron scales, although the net effect is overall target reduction with an average reduction of Fe3+ by 66%. Moreover, the data show that the physical state of the country rock (i.e., particulate vs. solid rock) has an effect on the magnitude of how the lightning-induced metamorphism effects are observed.