Paleomagnetic and micromagnetic measurements of Middle Devonian pillow lavas from Germany

Paleomagnetic data from the Middle Devonian are typically difficult to interpret. Directions and paleointensities often do not fit with dipolar field behavior or expected paleogeography. The reason why the geomagnetic field cannot be reconstructed with traditional methods has been topic of debate, but no consensus has been reached. We would like to understand what happened to the geomagnetic field during the Middle Devonian and why the configuration of the field was potentially unusual.

We aim to expand the existing paleomagnetic record for the Middle Devonian by sampling a site in Braunfels, Germany. This site consists of relatively unaltered pillow lavas. Petrographic and rockmagnetic analyses indicate the presence of magnetite and minor maghemite in the samples. We obtained paleomagnetic directions using alternating field (AF) and thermal demagnetization experiments. The directions are scattered and do not cluster around paleomagnetic directions that are expected for Germany in the Devonian.

Paleointensity data were acquired using the ZIIZP-Thellier method, resulting in a field intensity of approximately 6 µT, which equals a VADM of 8-15 ZAm². The latter is in line with very low field intensities generally reported for the Devonian.

Various mechanisms have been suggested to explain the typically scattered and ambiguous Devonian paleomagnetic data, such as significant overprinting, tectonic rotations and a non-dipolar field configuration. Our results confirm an extremely weak magnetic field, but this alone does not explain the scattered directions. To exclude the possibility that the scattered directions are related to (partial) overprinting we use Quantum Diamond Microscope imaging to assess the magnetizations of individual magnetic grains instead of the bulk magnetic signal of the sample. With this method, a distinction can be made between e.g., different generations of magnetizations, revealing information on the Middle Devonian geomagnetic field that was previously inaccessible by considering the magnetic moments of bulk samples alone.