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On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows: 2) An experimental case study

Verard, Christian, Roman Leonhardt, Michael Winklhofer, and Karl Fabian (2008), On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows: 2) An experimental case study, Phys. Earth Planet. Inter., doi:10.1016/j.pepi.2008.07.018.

Abstract
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 & Soffel, 2002). 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.
Further information
BibTeX
@article{id1322,
  author = {Christian Verard and Roman Leonhardt and Michael Winklhofer and Karl Fabian},
  journal = {Phys. Earth Planet. Inter.},
  month = {jul},
  title = {{On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows: 2) An experimental case study}},
  year = {2008},
  url = {http://dx.doi.org/10.1016/j.pepi.2008.07.018},
  doi = {10.1016/j.pepi.2008.07.018},
}
EndNote
%0 Journal Article
%A Verard, Christian
%A Leonhardt, Roman
%A Winklhofer, Michael
%A Fabian, Karl
%D 2008
%J Phys. Earth Planet. Inter.
%T On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows: 2) An experimental case study
%U http://dx.doi.org/10.1016/j.pepi.2008.07.018
%8 jul
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Printed 06. Dec 2019 12:26