Micromagnetic tomography in practice

Abstract

Methods to derive paleodirections or paleointensities from rocks currently rely on measurements of bulk samples (typically ~10 cc), the obtained magnetization is a result of the sum of the magnetization from all the differing grains within the sample. The process of recording and storing magnetizations as function of temperature, however, differs for grains of various sizes and chemical compositions. Within Fundamental rock-magnetism often single grains are analyzed using high-end techniques, so far the obtained information about the grains is only suitable for 2D interpretations. Here we set out to bridge the gap by; non-destructively asses the full magnetic vector of many individual grains within a ‘bulk ‘ sample. This is done by Micromagnetic Tomography. Firstly, the distribution and volume of the remanence carrying grains in the sample must be assessed; this is done using a MicroCT scanner capable of detecting grains >1 micron. Secondly, the magnetic stray field perpendicular to the surface of a thin sample is measured using a high-resolution Scanning SQUID Microscope. A mathematical inversion of these measurements yields the isolated direction and magnitude of the magnetic moment of individual grains in the sample. As the measured strength of the magnetic field decreases with the third power as function of distance to the exerting grain (as a result of decay in three dimensions), the magnetization of grains in the top 25 micrometers of the sample can be assessed reliably.