TESTING THE HYPOTHESIS OF A TWO-MODE GENERATION OF THE GEOMAGNETIC FIELD IN THE PROTEROZOIC IN THE CONTEXT OF NEW EXPERIMENTAL DATA ON ITS PALEOINTENSITY NEAR 1500 МILLION YEARS AGO
V.V. Shcherbakova1, A.M. Pasenko2, V.P. Shcherbakov1, G.V. Zhidkov1, N. A. Afinogenova1, A.A. Karimov3
1 Borok Geophysical Observatory, department of Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Yaroslavl obl., pos. Borok, Russia
2О.Yu. Schmidt United Institute of the 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. bimodal mode of geomagnetic field generation, low paleointensity, Proterozoic, Thellier-Coe and Wilson-Burakov methods, Anabar Uplift, Siberian Platform
Abstract
Paleointensity determinations of the geomagnetic field in the Proterozoic were performed on a collection of rocks selected from intrusive bodies in the northern part of the Siberian Platform, with an age of approximately ≈ 1500 Ma. The magnetic and thermomagnetic properties of the rocks have been studied in detail, and X-ray diffraction and electron microscopic studies have been performed. It has been shown that the carriers of the characteristic component of the natural remanent magnetization are single- and small pseudo-single-domain magnetite grains. The paleointensity Banc was determined using the Thellier-Coe method with the check-points procedure and the Wilson-Burakov method. For six sites, 22 (34 with doubles) determinations were obtained that meet modern reliability criteria. All of them show low values of the field strength and virtual dipole moment, varying within the range (4.7–17.6) μT and (1.21–3.85)×1022 Am2, respectively, which is about four times lower than their
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average values in the modern epoch. A joint analysis of the paleointensity determinations for the Proterozoic presented in the world paleointensity database and the inclination function a(I) = 1/[1+3cos2(I)]½ was performed. It is shown that the data in the diagram (Banc vs a(I)) fall into two clusters of high and low paleointensity values. At the same time, both clusters testify to the dipole geometry of the field, independent of its intensity. The performed analysis confirms the hypothesis of a bimodal mode of geomagnetic field generation in the Proterozoic, which may indicate the absence of a solid inner core in the Early and Middle Proterozoic, attributing its nucleation to a later time (Ediacaran).
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