Paleomagnetism of impact spherules from Lonar crater, India and a test for impact-generated fields

Planetary surfaces have been ubiquitously melted by meteoroid impacts throughout solar system history. The resulting impact melts form some of the youngest igneous samples from rocky bodies like the Moon, Mars, and asteroids. Upon cooling, these melts may record any ambient planetary magnetic fields as well as postulated transient fields generated by impact plasmas. Impact-generated fields have been proposed as a key alternative to the core dynamo hypothesis for the paleomagnetism of extraterrestrial bodies. Here we describe a paleomagnetic study of basaltic impact glasses from the Lonar impact crater situated in the Deccan Traps in Maharashtra, India. Previous theoretical work predicts extremely strong magnetic fields (possibly >1,000 times the Earth's surface field) may have been transiently generated during the Lonar impact. We find that the glasses contain a natural remanent magnetization (NRM) whose properties depend strikingly on sample mass. Small (<0.5 g), splash-form samples demagnetize erratically and are inefficiently magnetized, while larger, irregularly shaped samples contain a stable component that is efficiently magnetized similar to Lonar basalts. However, the rock magnetic recording properties of these samples are uncorrelated with mass. Therefore, we conclude that the size dependence of the NRM reflects a difference in how the samples acquired thermoremanence. The splash forms of the smaller samples indicate they cooled during flight and therefore that they were magnetized while in motion, explaining their weak and unstable NRM. This motional NRM is a new manifestation of thermoremanent magnetization not observed before in geologic samples. No glasses contain evidence for any strong (>~100 μT) impact-generated fields.