Deciphering Middle Devonian geomagnetic field behaviour through analysis of individual remanence carriers.

The behaviour of the geomagnetic field through time provides insight into the formation and evolution of Earth’s interior. However, for certain geological periods the Earth’s magnetic field behaviour is poorly understood. For instance, the Devonian period remains enigmatic as palaeomagnetic records of this time are ambiguous. Devonian bulk rock samples often yield scattered magnetic directions, even when ideal magnetic carriers are present. This may be explained by partial remagnetisation or by complex, non-dipolar magnetic field behaviour.

We conducted a palaeomagnetic study on Middle Devonian pillow basalts from the Rhenish Massif, Germany and encountered inconclusive bulk magnetic behaviour. Directions obtained through alternating field (AF) demagnetisation are scattered and do not cluster around a common mean. While some of these results can be attributed to partial overprints, not all findings can be explained with this mechanism. To investigate this further, we analysed the individual magnetic grain behaviour in the samples using Micromagnetic Tomography (de Groot et al., 2018; 2021).

With MMT, we measured the natural remanent magnetisation (NRM) of the samples, and we conducted an AF demagnetisation sequence. With this, we studied the individual grains in the samples across five demagnetisation states. This approach aimed to identify the magnetically stable grains in the samples that are capable of preserving interpretable geomagnetic signals.

Our findings may improve the understanding of Middle Devonian geomagnetic field behaviour and enable the use of previously uninterpretable samples in reconstructing long-term geomagnetic field behaviour.