Microspherules Formed by Lightning Strikes

Abstract Natural microspherules are formed by high‐temperature processes and are present throughout the geologic record to the present day. We report the discovery of large numbers of microspherules recovered from a rock pavement in the Pilbara region, Western Australia. Textures range from glassy to coarse‐grained, with many particles containing crystallites, vesicles, and relict grains. Compositions are non‐chondritic and are either dominated by silicates or Fe‐Ti‐bearing oxides. Spherule and relict grain compositions show strong affinities to the mineralogy of the underlying rock, a Paleoarchean granite gneiss. Bulk compositions suggest formation by a localized melting process with precursors dominated by individual pre‐existing minerals, with minimal mixing. Numerical modeling of the formation of spherules suggests formation by rapid quenching, possibly from melt droplets. Modeled cooling times are consistent with compositions that indicate limited evaporation. The compositions and textures of these spherules are not compatible with either microtektites formed by meteor impact or micrometeorites formed by the atmospheric entry of cosmic dust and are instead interpreted to have formed via lightning strikes. Spherules generated by lightning strikes may be present in the geologic record and thus could be used as a paleoclimate proxy where other signatures, such as main mass fulgurites, have not survived.