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.

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Our understanding of the behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g. lavas. Almost all experiments to determine the past state of the Earth’s magnetic field use bulk samples (typically 1 - 10 cc) and measure their magnetic moment after series of laboratory treatments. Lavas, however, consist of mixtures of different iron-oxide grains that vary in size, shape, and chemistry. Some of these grains are good recorders of the Earth’s magnetic field; others are not. Only a small amount of adverse behaved magnetic grains in a sample already hampers all classical experiments to obtain paleointensities; success rates as low as 10-20% are common, i.e. for 80-90% of all lavas vital information on paleointensities is lost before it can be uncovered. Recently, we showed that it is possible to determine the magnetization of individual grains inside a synthetic sample using a new technique: Micromagnetic Tomography. The individual magnetizations of grains are determined by inverting scanning magnetometry data from the surface on the sample onto the known locations, sizes and shapes of the magnetic grains that are obtained from a microCT scan of the sample. The synthetic sample used for our proof-of-concept, however, was optimized for success: the dispersion of magnetic markers was low, and the magnetite grains had a well-defined grain size range. Furthermore, the scanning SQUID microscope used requires the sample to be at 4 K, below the Verweij transition of the magnetite grains. Here we present the first Micromagnetic Tomography results from natural samples. We used two magnetic scanning techniques that operate at room temperature, a Magnetic Tunneling Junction set-up and a Quantum Diamond Magnetometer, to acquire the magnetic surface scans from a Hawaiian lava and calculated magnetic moments of individual grains present. We show that it is possible to acquire rock magnetic information as function of grain size from these natural samples and reveal the first results of interpreting a paleomagnetic direction from selected subsets of grains in our samples. These are the first steps towards deriving rock magnetic and paleomagnetic information from subsets of known good recorders inside lava samples, a technique that will revolutionize our field of research.@en

Micromagnetic Tomography is a new technique that is capable of determining the magnetizations of individual magnetic grains inside a sample. Ultimately this technique will enable us to derive paleomagnetic information from a subset of magnetic grains in a sample that are known good recorders while disregarding the contributions of ill-behaved grains. The proof-of-concept of Micromagnetic Tomography was provided using a synthetic sample that was optimized for success: the dispersion of magnetic markers was one to two orders of magnitude lower than in natural samples, i.e. lavas. To obtain reliable paleomagnetic information from a lava, however, we presumably need the magnetic moments of millions of grains from one sample. To determine the magnetic moments of as much particles as possible in one experiment it is paramount to understand the tradeoff between the thickness of the sample, the resolution of the magnetic scan and the dispersion of magnetic markers in a sample. Since the magnetic measurements are a surface scan with a set resolution, the amount of magnetic grains included in each data point is governed by the thickness of the sample. A thicker sample has more magnetic grains per surface area and theoretically more grains could be inverted for from one scan. For each magnetic grain three variables must be solved by the inversion: the magnetization in the three orthogonal axes with respect to the reference frame. To ensure that the inversion is overdetermined, the number of variables to solve must be much less than the number of data points in the magnetic surface scan. The sample can therefore not be infinitely thick. Moreover, the magnetic moment of deeper grains in the sample is harder to resolve due to a weaker contribution of these grains to the surface magnetization. To assess the optimal thickness of the sample for the current Micromagnetic Tomography routine we inverted the results of natural samples that are 30, 60, 90, 120 and 150 µm thick. Here we show the results of these experiments and determine the optimal thickness of a lava sample for Micromagnetic Tomography. To obtain reliable paleomagnetic information using Micromagnetic Tomography it is sufficient to invert a surface area of approximately 1-2 cm2, which is well within the possibilities of the current QDM set-up.@en

Obtaining reliable information from even the most challenging paleomagnetic recorders, such as the oldest igneous rocks and meteorites, is paramount to open new windows into Earth's history. Currently, such information is acquired by simultaneously sensing millions of particles in small samples or single crystals using superconducting quantum interference device magnetometers. The obtained rock-magnetic signal is a statistical ensemble of grains potentially differing in reliability as paleomagnetic recorder due to variations in physical dimensions, chemistry, and magnetic behavior. Here we go beyond bulk magnetic measurements and combine computed tomography and scanning magnetometry to uniquely invert for the magnetic moments of individual grains. This enables us to select and consider contributions of subsets of grains as a function of particle-specific selection criteria and avoid contributions that arise from particles that are altered or contain unreliable magnetic carriers. This new, nondestructive, method unlocks information from complex paleomagnetic recorders that until now goes obscured.

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