Micromagnetics and magnetomineralogy of ultrafine magnetite inclusions in the Modipe Gabbro



[1] Iron oxide inclusions in pyroxene crystals from the Modipe Gabbro, Botswana, have been studied to determine their recording fidelity. Hysteresis parameters, first-order reversal curves, isothermal remanent magnetization acquisition curves, and thermomagnetic data all indicate that the iron oxide occurs as stoichiometric magnetite in the form of single-domain and/or small pseudo single-domain particles. Using distributions for the grain shape and nearest-neighbor spacings determined from optical micrographs as input, a first-order reversal curve (FORC) diagram was simulated using a numerical micromagnetic model. The simulated FORC diagram was found to closely match the measured FORC distributions. Analysis of the interaction fields in the model found that the standard deviation of the interaction field distribution was 2.7 mT compared to a bulk coercivity of ~20 mT, suggesting that the majority of particles are unlikely to be affected by magnetostatic interactions. Thellier analysis of the samples induced with an initial laboratory thermoremanence yielded near-perfect behavior. It is suggested that the Modipe Gabbro dated at 2784.0 ± 1.5 (2σ) Ma is potentially an ideal recorder of the geomagnetic field.