On the possibility of recovering paleo-diurnal magnetic variations in transitional lava flows

One of the tenets in paleomagnetism is that perturbations of the ground magnetic field due to magnetospheric or ionospheric current systems are too small to leave a detectable paleomagnetic signature in lava flows. As suggested by recent work in paleomagnetosphere modelling, however, external-field perturbations may be significantly enhanced during periods of transitional field behavior, particularly when the dipole-field axis is strongly tilted towards the equator, which then leads to an extremely dynamic magnetosphere on the diurnal time scale even for quiet solar wind conditions. We here demonstrate that thin (rapidly cooled) lava flows (∼50 cm thick) with high magnetic blocking temperatures (within ∼50-100 ° C below the Curie temperature) indeed have the potential to record such diurnal perturbations. Further, an experimental protocol is suggested to paleomagnetically extract these perturbations. Our proof-of-concept is based on numerical modelling of thermoremanence (TRM) acquisition and simulation of thermal demagnetization surfaces for discrete temperature steps in function of vertical position in the flow. The TRM direction recovered at a given thermal demagnetization step varies with vertical position in the flow and thereby reveals the wave form of the external-field variation. Characteristically, the vertical position of a captured signal changes systematically with unblocking temperature, which reflects the oblique orientation of cooling isochrons, along which the signals are blocked. The signals have their largest amplitudes at the maximum unblocking temperatures, but decay away at lower temperatures. It is by these systmatic trends that external-field perturbations, if trapped, can be paleomagnetically identified and distinguished from a secondary overprint. The experimental procedure requires a sample spacing of 1 cm (with 1 cm drill cores) and small thermal demagnetization steps (15 °C) at elevated temperatures.