Insights on lightning-induced remanent magnetization from high-current impulse experiments - vs.2

Lightning strikes can induce strong and complex remanent magnetizations in rocks, potentially overprinting their natural paleomagnetic record. To investigate the mechanisms and outcomes of lightning-induced remanent magnetization (LIRM), we conducted controlled high-current impulse experiments on mafic and ultramafic rocks using the lightning generator of the High Energy Laboratory at the University of São Paulo. Peak currents up to ~80 kA with waveform reversals were applied on alkaline volcanic rocks from Trindade Island, Vista Alegre tholeiites, glimmerites and carbonatic breccias from Planalto da Serra, and Quatipuru basalts. Magnetic susceptibility, hysteresis, FORC diagrams, and Lowrie–Fuller tests were measured before and after the experiments. Results show that lightning-like currents increase remanent magnetization in most lithologies, with some specimens exhibiting >400% enhancement, and REM ratios (NRM/SIRM) rising above 0.1, consistent with natural fulgurites and lodestones. Importantly, waveform polarity reversals produced opposite magnetization directions between nearby and more distant specimens, resulting in the coexistence of two components of remanence within tens of centimeters of distance from the striking point. Quatipuru basalts, which are predominantly paramagnetic, exhibited distinct behavior, including susceptibility decreases and an enhanced frequency-dependent response, suggesting the formation of superparamagnetic grains. These findings demonstrate that lightning discharges can significantly modify the magnetic properties of rocks. Additionally, lightning discharges can produce opposite-direction remagnetization in low-coercivity assemblages, expressed as a characteristic curvature in AF demagnetization paths. In some cases, LIRM intensity is lower than that produced by a 100 mT laboratory IRM despite stronger lightning-generated fields.  Lightning-induced changes provide diagnostic signatures, amplifying their remanence in orders of magnitude while generating complex directional patterns, which complicate paleomagnetic interpretations.