Paleointensity derived from igneous rocks of Kamchatka volcanoes of the Late Pleistocene-Holocene epoch

This work is devoted to paleomagnetic studies of lava samples from three volcanoes of Kamchatka in order to define the age of lava flows and to obtain data of paleosecular variations of the geomagnetic field for the Kamchatka region. We studied 53 samples from 7 sites from lava flows of the Avachinsky, the Gorely and the Tolbachik volcanoes. The study of paleosecular variations recorded in the magnetization of the lava flows of volcanoes makes it possible to create a magnetochronological scale for epochs of the same polarity.

According to the data of electron microprobe and thermomagnetic analyzes, the magnetic properties of samples from the lava flows of the Avachinsky volcano are mainly determined by titanic magnetite with a Curie temperature Tc = (540-580) °С. The study of magnetic mineral grains using electron and magnetic force microscopy showed the presence of decay structures in grains, indicating the high-temperature oxidation of titanomagnetite. Ferrimagnetic grains of samples from the Gorely and Tolbachik volcanoes are represented by titanomagnetite with a Curie temperature Tc = (200–300) °C. According to the hysteresis characteristics, the magnetic structure of the grains corresponds to a single-domain and pseudo-single-domain state. Thermal and magnetic cleanings showed the predominance of one component in the NRM. The geomagnetic field intensity was determined by the Thellier method in the Coe modification.

It was found that the paleointensity value H_anc = 55±3 μT, determined from the NRM of samples of the 2012 eruption from the Tolbachik volcano, differs from the modern magnetic field in the area of this volcano by the IGRF-12 model by only 4% (Н_IGRF = 53 μT). This indicates the reliability of our methodology for determining paleointensity from the most stable part of the NRM of igneous rocks.

A comparison of the coordinates of the paleomagnetic pole (N 66º±4º, E 266º±5º) and the virtual dipole magnetic moment of the Earth (VDM = 8.3±0.9*10²² A*m²) with data on variations of the geomagnetic field over the past 10,000 years [Burlatskaya, 2007; McElhinny, 1982] allows us to conclude that the investigated lava flow belongs to the historical eruptions of 1827. The coordinates of the virtual geomagnetic pole (N 83º±3º, E 254º±21º) and the value of VDM = 8.0±0.3*10²² A*m² determined from the samples belonging to the second lava flow of the Avachinsky volcano indicate that rocks are formed in the result of the eruption, which occurred 5-5.5 thousand years ago.

It was revealed that the magnitude (H_anc =65±5μT) and the direction of paleointensity determined by the NRM of the samples from Gorely volcano significantly differ from the characteristics of the modern magnetic field. The assumption is made that the studied samples belong to the outpouring of lava, which occurred about 2.7 thousand years ago, during the "Sterno-Etrussia" geomagnetic excursion.

 

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.