Ultra-Low Geomagnetic Field Intensity in the Mesoproterozoic Based on the Dike Complex Rocks of the Bunger Oasis (1133 Ma, East Antarctica)

–In order to determine the paleointensity of the geomagnetic field in the Precambrian, a collection of rocks from 83 samples of 19 sites selected from the Mesoproterozoic (1133 Ma) dikes of the Bunger Oasis (East Antarctica) was studied. The magnetic and thermomagnetic properties of rocks have been studied in details, the domain structure of magnetic grains, carriers of natural remanent magnetization, has been evaluated, and micromagnetic and X-ray studies have been carried out. It is shown that the carriers of the characteristic component of natural remanent magnetization are single- and/or small pseudo-single-domain grains. Paleointensity was determined by the Thellier-Coe method with the check-points procedure and, in addition to it, the express Wilson method. For 11 sites, low to extremely low values of paleointensity B anc were obtained, which meet modern reliability criteria, the main array of B anc determinations by sites is grouped around (1.5–5) μT. The calculated values of the virtual dipole moment VDM vary from 3 to 8 (×10 21 Am 2 ), which is an order of magnitude (and more) less than the current VDM value. Such low values of paleointensity do not agree with the hypothesis of an increased intensity of the geomagnetic field in the Middle Proterozoic and testify against the hypothesis of the formation of the Earth’s inner core in the time interval from 1.5 to 1 Ga ago. An analysis of the VDM values presented in 2022 in the world database over the interval of 3600–500 Ma showed that the behavior of the geomagnetic field in the Precambrian is characterized by the alternation of long periods of low and high intensity with a tendency to its gradual weakening from the Archean to the Neoproterozoic. A linear approximation of such a dependence of the VDM value on time has a correlation coefficient of 0.22 at a confidence level of 0.98. The noted features of the behavior of the intensity of the geomagnetic field in the Proterozoic and Paleozoic testify in favor of the hypothesis of the origin of the Earth’s solid core not earlier than the Late Neoproterozoic.