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Winklhofer, Michael; Fabian, Karl; Leonhardt, Roman; Verard, Christian (2008): On the possibility of recovering paleo-diurnal magnetic variations in transitional lava flows I. Constraints from thermoremanence modelling for an experimental protocol. In: Physics of the Earth and Planetary Interiors, Vol. 169, No. 1-4, SI: pp. 108-116
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One of the tenets in paleomagnetism is that perturbations of the groundmagnetic field due to magnetospheric or ionospheric current systems aretoo 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 duringperiods of transitional field behavior, particularly when thedipole-field axis is strongly tilted towards the equator, which thenleads to an extremely dynamic magnetosphere on the diurnal time scaleeven for quiet solar wind conditions. We here demonstrate that thin(rapidly cooled) lava flows (similar to 50 cm thick) with high magneticblocking temperatures (within similar to 50-100 degrees C below theCurie temperature) indeed have the potential to record such diurnalperturbations. Further, an experimental protocol is suggested topaleomagnetically extract these perturbations. Our proof-of-concept isbased on numerical modelling of thermoremanence (TRM) acquisition andsimulation of thermal demagnetization surfaces for discrete temperaturesteps in function of vertical position in the flow. The TRM directionrecovered at a given thermal demagnetization step varies with verticalposition in the flow and thereby reveals the wave form of theexternal-field variation. Characteristically, the vertical position of acaptured signal changes systematically with unblocking temperature,which reflects the oblique orientation of cooling isochrons, along whichthe signals are blocked. The signals have their largest amplitudes atthe maximum unblocking temperatures, but decay away at lowertemperatures. It is by these systmatic trends that external-fieldperturbations, if trapped, can be paleomagnetically identified anddistinguished from a secondary overprint. The experimental procedurerequires a sample spacing of 1 cm (with 1 cm drill cores) and smallthermal demagnetization steps (15 degrees C) at elevated temperatures.