کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
4742638 1641578 2008 14 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات فیزیک زمین (ژئو فیزیک)
پیش نمایش صفحه اول مقاله
On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows
چکیده انگلیسی
Geomagnetic field variations of external origin may be enhanced during periods of transitional field behaviour, particularly when the dipole moment is low, in which case they are likely to leave a paleomagnetic signature in rapidly cooled lava flows. To test this proposition, we have resampled en bloc and studied in fine detail a thin transitional Aa flow from a mid-Miocene lava sequence on Gran Canaria which was paleomagnetically investigated previously (Leonhardt, R., Soffel, H.-C., 2002. A reversal of the Earth's magnetic field recorded in mid-Miocene lava flows of Gran Canaria, Paleointensities. Journal of Geophysical Research 107, 2299. doi:10.1029/2001JB000949). The flow is characterised by high-unblocking temperatures, an equatorial VGP position and a very low absolute palaeointensity of ∼2 μT. Two slabs were cut out of the flow and sampled at 1 cm intervals, along four vertical profiles running parallel to each other. Thermal demagnetisation was performed on two profiles using heating steps as small as 15 °C at elevated temperatures. The high-temperature part of the unblocking spectrum was found to be remarkably constant across the flow, as was the Curie temperature of 540 °C, and the negligible anisotropy of magnetic susceptibility. The exsolution lamallae observed under the microscope point to deuteric (high temperature) oxidation having occurred prior to the acquisition of the primary thermoremanent magnetisation. While the absolute palaeointensity values vary only little with vertical position, the magnetisation directions recovered by thermal demagnetisation vary considerably (on average, by some 20° at 500 °C). These large variations can be attributed to an overprint by secondary minerals, formed by fluid diffusion around vesicles and low-temperature oxidation. Since the secondary magnetisation recorded transitional directions as well, the overprint must have occurred soon after emplacement. The directional variations typically decrease in amplitude with increasing blocking temperature, which is contrary to what would be expected if pronounced diurnal external field variations were trapped in the flow.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Physics of the Earth and Planetary Interiors - Volume 169, Issues 1–4, August 2008, Pages 117-130
نویسندگان
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