How Micromagnetic Tomography deals with MicroCT Limitations: The Grain Size Distribution of Magnetite in Hawaiian Basalts

Micromagnetic Tomography (MMT) is a new technique that determines magnetic moments of individual grains embedded in a sample. Individual magnetic moments are obtained through a non-destructive method: the position of magnetic carriers is found through micro-X-ray computed tomography (MicroCT) and the magnetic surface flux signal of a sample is obtained by employing a high-resolution magnetometer, such as a Quantum Diamond Magnetometer. By carefully selecting which grains to interpret and which grains not, MMT can open new possibilities in interpreting the palaeomagnetic information stored in the most enigmatic magnetic carriers. However, MMT needs to overcome important hurdles before it can reach this ultimate goal. One of these challenges is the limitations in the resolution of MicroCT: PSD grains <0.75 µm are often not detected although they produce a magnetic signal and may be omni-present in our samples. Missing these grains is a source of error in the least squares inversion protocol that calculates magnetic moments.

Here, we characterize the grain size distribution of iron-oxides in a basalt with a resolution from 20 nm upwards using a combination of focused ion beam scanning electron microscopy (FIB-SEM), SEM, and MicroCT. Fitting a lognormal distribution to this dataset, reveals a peak in grain sizes of 70 to 80 nm. Then we use this grain size distribution to characterize the effect of grains that are missed by the MicroCT measurements using numerical MMT simulations.

First, we populate numerical MMT models with a realistic number of iron-oxides with grain sizes from 40 nm onwards to fully cover the SD, PSD, and MD grain size domain. Then, we calculate the resulting magnetic signal at the location of the magnetic sensors, and apply the inverse procedure using only the position of grains >0.75 µm, which is the detection limit of the routinely used MicroCT machine in MMT studies to date. A comparison of the original and calculated magnetic moments of individual grains results in an estimation of the magnetic moment error of undetected grains, i.e., the impact of the 'missing grains effect'. We observe that the larger grains (>1 µm) are solved reasonably well, but for smaller grains the MMT inversions fail to produce reliable results. Increasing the resolution of the MicroCT scans is therefore paramount.