Technique

Concept

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.

Machines

The MMT technique determines the magnetic moments of individual grains in a sample by inverting a two-dimensional magnetic surface scan of the sample based on the known locations and shapes of the iron-oxide grains as determined by a MicroCT scan of the sample. The input for any MMT experiment is thus (a) a magnetic surface scan and (b) a MicroCT characterization of the sample (FigureĀ 1). These two data sets must first be co-registered into a common spatial coordinate system before a mathematical inversion of the magnetic surface scan constrained by the MicroCT data can produce the magnetic moments of the grains. The accuracy of the inversion results can then be assessed by determining the residuals left by the inversion.