Fulgurites: the Earth’s minute melts and their interaction with the atmosphere

I.N. Bindeman, C. Cimarelli, J. Palandri

Abstract

We here report the first triple oxygen isotope analyses on a collection of natural and syntheticfulgurites generated in natural lightning and high current experiments, and fulgurites generated by electrical power line accidents. The goal is to investigate whether fulgurites experience very high Δ’¹⁷O due to UV photolysis or record rapid exchange with high δ¹⁸O, low Δ’¹⁷ O air-O₂. Experiments used a variety of pristine materials spanning from dunite to rhyolitic bulk rock composition and monomineralic materials like quartz and apatite. All experimental and natural fulgurites from igneous bedrock are anhydrous and exhibit moderate −0.02 to −0.04‰ Δ’¹⁷ O negative shifts (or 4–8%) toward air -O₂, regardless of their bulk composition. Several tektites of similar ∼2-15 mm size, analysed for comparison, show more substantial −0.2‰ negative shifts, ∼40% toward air -O₂. Fulgurites formed by fallen powerlines on wet soil are highly vesicular and they exhibit both positive and negative 0.02‰ Δ’¹⁷ O shifts, indicating interaction with both lowΔ’¹⁷ O air -O₂ and high-Δ’¹⁷O meteoric waters during formation and rapid (minutes long) cooling after melt formation. It appears that neither ground nor suspended particle experimental lightning produces unexpected photolytic mass independent ¹⁷O/¹⁸O/¹⁶O fractionation effects, in contrast to UV photolysis. Fulgurites in the geologic record can be further explored as monitors of O₂ pressure in CO₂ rich ancient atmospheres via their Δ’¹⁷O values, even if devitrified.