Verard, Christian; Leonhardt, Roman; Winklhofer, Michael; Fabian, Karl
On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows 2. An experimental case study.
In: Physics of the Earth and Planetary Interiors, Vol. 169, Nr. 1-4: S. 117-130
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Geomagnetic field variations of external origin may be enhanced duringperiods of transitional field behaviour, particularly when the dipolemoment is low, in which case they are likely to leave a paleomagneticsignature in rapidly cooled lava flows. To test this proposition, wehave resampled en bloc and studied in fine detail a thin transitional Aaflow from a mid-Miocene lava sequence on Gran Canaria which waspaleomagnetically investigated previously (Leonhardt, R., Soffel, H.-C.,2002. A reversal of the Earth’s magnetic field recorded in mid-Miocenelava flows of Gran Canaria, Paleointensities. journal of GeophysicalResearch 107, 2299. doi:10.1029/2001JB000949). The flow is characterisedby high-unblocking temperatures, an equatorial VGP position and a verylow absolute palaeointensity of similar to 2 mu T. Two slabs were cutout of the flow and sampled at I cm intervals, along four verticalprofiles running parallel to each other. Thermal demagnetisation wasperformed on two profiles using heating steps as small as 15 degrees Cat elevated temperatures. The high-temperature part of the unblockingspectrum was found to be remarkably constant across the flow, as was theCurie temperature of 540 degrees C, and the negligible anisotropy ofmagnetic susceptibility. The exsolution lamallae observed under themicroscope point to deuteric (high temperature) oxidation havingoccurred prior to the acquisition of the primary thermoremanentmagnetisation. While the absolute palaeointensity values vary onlylittle with vertical position, the magnetisation directions recovered bythermal demagnetisation vary considerably (on average, by some 20degrees at 500 degrees C). These large variations can be attributed toan overprint by secondary minerals, formed by fluid diffusion aroundvesicles and low-temperature oxidation. Since the secondarymagnetisation recorded transitional directions as well, the overprintmust have occurred soon after emplacement. The directional variationstypically decrease in amplitude with increasing blocking temperature,which is contrary to what would be expected if pronounced diurnalexternal field variations were trapped in the flow.