Micromagnetic Tomography (MMT) is a new technique that allows to determine magnetic moments of individual grains in volcanic rocks. Current MMT studies either showed that it is possible to obtain magnetic moments of relatively small numbers of grains in ideal sample material, or provided important theoretical advances in MMT inversion theory and/or its statistical framework. Here we present a large-scale application of MMT on a sample from the 1907-flow from Hawaii’s Kilauea volcano producing magnetic moments of 1,646 individual grains. To assess the robustness of the MMT results, we produced 261,305 individual magnetic moments in total: an increase of three orders of magnitude compared to earlier studies and a major step towards the number of grains that is necessary for paleomagnetic applications of MMT. Furthermore, we show that the recently proposed signal strength ratio is a powerful tool to scrutinize and select MMT results. Despite this progress, still only relatively large iron-oxide grains with diameters >1.5-2 µm can be reliably resolved, impeding a reliable paleomagnetic interpretation. To determine the magnetic moments of smaller (< 1 µm) grains that may exhibit PSD behavior and are therefore better paleomagnetic recorders, the resolution of the MicroCT and magnetic scans necessary for MMT must be improved. Therefore, it is necessary to reduce the sample size in future MMT studies. Nevertheless, our study is an important step towards making MMT a useful paleomagnetic and rock-magnetic technique.

Our understanding of the past behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g. (volcanic) rocks. Bulk rock samples, however, often contain magnetic grains that differ in chemistry, size and shape; some of them record the Earth’s magnetic field well, others are unreliable. The presence of a small amount of adverse behaved magnetic grains in a sample may already obscure important information on the past state of the geomagnetic field. Recently it was shown that it is possible to determine magnetizations of individual grains in a sample by combining X-ray computed tomography and magnetic surface scanning measurements. Here we establish this new Micromagnetic Tomography (MMT) technique and make it suitable for use with different magnetic scanning techniques, and for both synthetic and natural samples. We acquired reliable magnetic directions by selecting subsets of grains in a synthetic sample, and we obtained rock-magnetic information of individual grains in a volcanic sample. This illustrates that MMT opens up entirely new venues of paleomagnetic and rock-magnetic research. MMT’s unique ability to determine the magnetization of individual grains in a nondestructive way allows for a systematic analysis of how geological materials record and retain information on the past state of the Earth’s magnetic field. Moreover, by interpreting only the contributions of known magnetically well-behaved grains in a sample MMT has the potential to unlock paleomagnetic information from even the most complex, crucial, or valuable recorders that current methods are unable to recover.