Paleomagnetic Recording at the Grain Scale

Natural rocks harbor diverse assemblages of magnetic mineral grains that record information about past dynamo activity and plate motions, among other processes. For much of its history, however, the field of paleomagnetism has counted on a thorough theoretical understanding of only very fine (≤100 nm) grains magnetized during heating. Here we review experimental and computational advances to move beyond this limitation. Magnetic field microscopy allows us to physically identify mineral grains carrying specific paleomagnetic signals, while nanotomography coupled with micromagnetic simulations offers, for the first time, a quantitative picture of how most naturally occurring magnetic grains behave across geologic time. Together, these techniques open the door to retrieving records from less-thanideal rocks with complex geological histories.