Magnetism is one of the most powerful tools available to geoscientists trying to unravel our planet’s history. Studies of Earth’s magnetic field and of magnetic nanoparticles in natural environments, which can yield information regarding the evolution of our planet’s atmosphere, surface environment, and interior, depend on reliable methods to characterize magnetic minerals. For the past 40 years, the Day diagram has been widely used to classify magnetic mineral assemblages into three distinct domains, including single-domain particles capable of retaining a high-fidelity record of Earth’s magnetic field for billions of years and multidomain particles whose paleomagnetic properties are not stable over geologic timescales.
Now Roberts et al. argue that interpretations of Day diagrams are complicated by no less than 10 issues and are therefore too ambiguous to be used to discriminate between these domains without substantial additional information. One such issue is magnetic mineralogy: Because the particle size at which domain changes occur varies widely for different minerals, comparing data for more than one magnetic mineral within the same sample creates significant complications. The researchers also discuss variations in the shape of single-domain particles, magnetostatic interactions, and the extent of surface oxidation, which can likewise exert crucial controls on the domain state of various magnetic minerals.
The authors’ analysis suggests that each of the biasing factors they examined can cause large variations in the distribution of data in Day diagrams. Because many of these issues are not well understood or constrained with respect to geological materials, they argue that this underlying ambiguity undermines the use of Day diagrams to diagnose sample particle size, mineralogy, and domain states and that there is an urgent need to adopt new approaches for diagnosing the properties of magnetic mineral assemblages. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015247, 2018)
—Terri Cook, Freelance Writer