TESTING THE HYPOTHESIS OF BIMODAL GEOMAGNETIC FIELD IN THE PROTEROZOIC WITHIN THE CONTEXT OF NEW EXPERIMENTAL DATA ON ITS PALEOINTENSITY ABOUT 1500 MA
V.V. Shcherbakova1, A.M. Pasenko2, V.P. Shcherbakov1, G.V. Zhidkov1, N.A. Afinogenova1, A.A. Karimov3
1Borok Geophysical Observatory of the Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Borok 142, Russia
2Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
3Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Proterozoic, low paleointensity, bimodal geomagnetic field in the Proterozoic, Thellier-Coe method, Wilson-Burakov method, Anabar uplift, Siberian platform
Abstract
Paleointensity determinations of the geomagnetic field in the Proterozoic were obtained using a set of rocks sampled from intrusive bodies in the north of the Siberian platform aged ~1500 Ma. Magnetic and thermomagnetic properties of rocks were studied in detail. X-ray diffraction and electron microscopic analyses were performed. It is shown that single- and small pseudo-single-domain magnetite grains serve as carriers of the characteristic natural remanent magnetization component. Paleointensity B anc was determined using the Thellier-Coe method with the checkpoint procedure (test heating to lower temperatures) and the Wilson-Burakov method. A total of 22 determinations (34 with duplicates) satisfying modern reliability criteria were obtained for six sites. All of them exhibit low values of the field magnitude and virtual dipole moment. They change within a range of (4.7-17.6) μT and (1.21-3.85) ×1022 Am2, respectively, which is approximately four times lower than their mean values in the modern era. Paleointensity determinations for the Proterozoic presented in the international database and the inclination function a ( I ) = 1/(1 + 3cos2(I))1/2 were subjected to a joint analysis. It is revealed that the data on the diagram ( B anc, a(I)) are divided into high- and low-paleointensity clusters. At the same time, both clusters indicate a dipole field geometry regardless of intensity. The analysis confirms the hypothesis of the bimodal geomagnetic field generation regime in the Proterozoic, which may indicate the absence of a solid inner core in the early and middle Proterozoic, attributing its formation to a later era (Ediacaran).
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