The Magnetic and Mineralogical Properties of the Stardalur Volcano, Iceland

Abstract

The Stardalur volcano is located 20 km northeast of Reykjavik on the Esja Peninsula, Iceland. The volcano consists of Quaternary lavas which have revealed a high magnetic anomaly in both ground- and aeromagnetic surveying, due to extremely strong natural remanent magnetizations (NRM). The mean NRM intensities are 61 A/m, with the highest intensity measured up to 128 A/m. These values are exceptional compared to the average for Icelandic basalts, which are known to be 4 A/m (Kristjansson, 2013). In order to understand the high and variable NRM, we completed different measurements for 60 samples from a drill core traversing 138 meter across the Stardalur lava flows. We used a vibrating sample magnetometer (VSM) to measure hysteresis properties, first order reversal curve (FORC) diagrams, and nonlinear Preisach maps. Susceptibilities were measured at low-, high and room temperature to calculate the Curie temperatures and magnetic mineral composition. These measurements prove that near end-member magnetite is the primary magnetic carrier. We did optical and scanning electron microscopy (SEM) to look at the oxides and microstructures within. Magnetite content was detected both by saturation magnetization (Ms) values obtained by the VSM and image analysis of SEM images, displaying a mean magnetite content of 2.2 %, slightly above other Icelandic basalts of 1-2 % (Ravilly et al., 2001). A significant amount of these magnetites show very small grain sizes (<1 μm), and are mainly of dendritic shape, indicating rapid crystallization. In addition, most magnetite grains show fine oxy-exsolution of ilmenite. The high amount of small grain sizes, as well as microstructures both leads to more single-domain to pseudo-single domain behaviour, which is thought to be an important contributor to the high remanence. To test this theory, we used micromagnetic modelling to investigate the domain state of the individual magnetite grains and to study the predicted hysteresis properties in the magnetite and magnetic interactions between particles. These models reveal that the small-sizes magnetite grains as well as oxyexsolutions are most likely responsible for the high NRM values.