The first definition of paleointensity in the Early Cretaceous basalts from the Franz Josef Land

We present the first definition of paleointesity of the Earth’s magnetic field that were obtained in the Early Cretaceous igneous rocks from the Franz Josef Land archipelago (Hooker and Scott Kelty Islands). The age of magmatism was determined by U-Pb method as the Early Cretaceous, about 125 Ma. A mean paleomagnetic direction for these rocks was calculated as D=40.2 deg, I=75.5 deg, a95=2.1 deg, k=89.3, N=52. A corresponding paleomagnetic pole is now located at Plat=69.0 deg; Plon=180.3 deg, A95=3.7 deg. An assessment of the domain structure of ferrimagnets using the Day plot diagram shows that the carriers of the natural remanent magnetization are pseudo-single-domain grains of titanomagnetites with varying Ti-content. Magnetic remanence was unblocked in temperatures of 350-400 °C. Some samples are characterized by unblocking temperatures of 560 °C. The determinations of the absolute values of paleointensity were obtained by the Thellier-Coe method with the implementation of the procedure "check-points". The values of B_anc vary within 8.4–16 µT, which is noticeably lower than the current magnetic field at the sampling point ≈55 µT. The corresponding VDMs of 1.13–2.25 × 10²² Am², with the current value of VDM ≈8 × 10²² Am². Numerous basalt flows are well studied by paleomagnetic and rockmagnetic methods, together with a large number of geochronological definitions, this makes basalts from the Franz Josef Land promising for obtaining new qualitative determinations of paleointensity in the Early Cretaceous time.

This work was supported by the RSF (project no. 19-17-00091) and the RFBR (project nos. 18-35-00273, 18-05-70035).