Variations of magnetic properties in thin lava flow profiles: Implications for the recording of the Laschamp Excursion

Two blocks have been cut in two lava flows from the Skalamaelifell Hill (Iceland) known to have recorded the Laschamp magnetic excursion (40.4 ± 2.0 ka). Detailed sampling and analyses have revealed multiple magnetic components. The high temperature/coercivity component corresponds to the primary magnetisation, with corresponding pole position close to the equator in the Pacific Ocean (φ = 251.90°/λ = −06.49°; dp = 0.74°/dm = 2.12°) and palaeo-intensity determinations below 5 μT. The different VGPs, however, vary in relation with the position of samples in the profiles. It could not be firmly established whether this distribution is associated with a change in the Earth magnetic field during lava cooling. In any case, variations are related with zones in the profiles marked, in particular, by the presence of vesicles. Moreover, the other components are interpreted to be linked with alteration inside the rocks, caused by interactions between vesicles content and the surrounding matrix. Secondary component, however, is interpreted as recording an excursional magnetic field, and should be of greater consideration in studies of Earth magnetic field excursions or reversals.

► Detailed paleodirection and paleointensity determination of the Laschamp Excursion. ► We discuss potential effects on palaeomagnetic record. ► Potential change in Earth magnetic field during lava flow cooling is considered. ► Chemical alteration between vesicles and the surrounding matrix led to overprinting. ► Overprints record transitional Earth magnetic field.