A missing palaeomagnetic signal in Middle Devonian pillow lavas

The configuration of the Earth's magnetic field during the Middle Devonian (394.3–378.9 Ma) is poorly understood. The magnetic signals in Middle Devonian rocks are often overprinted during the Kiaman reverse superchron, obscuring their primary remanence. In other cases, available palaeomagnetic data are ambiguous, conflicting with tectonic reconstructions or dipolar geomagnetic field behaviour. Here, we study the palaeomagnetic signal of Middle Devonian pillow basalts from the Rhenish Massif in Germany. Our rock-magnetic experiments show that the pillow basalts can store and retain magnetizations over time. However, the pillow basalts have a somewhat low initial natural remanent magnetization (NRM), which is not expected based on their magnetite content. The palaeomagnetic directions determined from alternating field demagnetization, thermal demagnetization and a combination of both, fail to cluster around a common mean. Great circle analyses of these palaeomagnetic directions reveal traces of both Kiaman and present-day field overprints. Our palaeointensity measurements have a very low success rate of < 2 per cent, with only one sample yielding a result of 5.9 μT. This low intensity might explain the low initial NRM of the samples and the lack of interpretable directional data in this study. However, given the very low success rate, this result does not convincingly represent the palaeointensity of the Middle Devonian field. All together, the lack of signal in our Middle Devonian pillow lavas could be a sign of an (ultra-)low, or non-dipolar or possibly even absent geomagnetic field during the time of formation.