Low paleomagnetic field in the proterozoic: new 1.72–1.76 ga paleointensity data obtained on the proterozoic volcanics from the Ukrainian shield

Abstract

A collection of igneous rocks from the Ukrainian Shield sampled from the Korsun-Novomyrhorod pluton (age interval 1760–1735 Ma, Ingul Domain) and from the Korosten pluton (age 1760–1750 Ma, North-Western Domain) is studied. To obtain reliable determinations of paleointensity (Banc), the magnetic and thermomagnetic properties of samples were studied and X-ray diffraction analyses were carried out. It is shown that the carriers of the characteristic component of natural remanent magnetization are single- and small pseudo-single-domain magnetite grains. To determine Banc, two methods were used: the Thellier‒Coe procedure with the pTRM-checks and the Wilson method. Paleointensity determinations are obtained from five sites and are shown to meet quality criteria. For all five sites, the values of the Banc and the virtual dipole moment (VDM) are extremely low, varying within the range of 3.6–9.76 μT and (0.92–2.43)×1022 Am2, respectively. The analysis of the data from the paleointensity world database (WDB) has shown that the operating mode of the geodynamo in the Proterozoic can be characterized by a succession of strong and weak dipole regimes, but the reality of this conclusion entirely depends on the reliability of the data reported in the literature and presented in the WDB.