Rock Magnetic Properties of Pleistocene Tephras from the Polovinka Section of the Central Kamchatka Depression

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Tephra deposits are important stratigraphic markers, particularly those originating from large explosive eruptions, such as documented from Kamchatka volcanoes. This study is focused on the magnetic characteristics of the Late Pleistocene tephras from the Polovinka outcrop. The aim is to detect possible peculiarities in the rock magnetic properties of different tephras, in order to evaluate their suitability for tephrochronological and correlation purposes. The Polovinka section is located on the right bank of the Kamchatka River in the Central Kamchatka depression (158° 55.670′E, 54° 54.654′N). For this study, twenty tephra layers from Middle to Late Pleistocene sediments were studied. The geochemical analyses indicate that the tephras are composed of basaltic-andesite, andesite, dacite and rhyolite. The magnetic mineral fraction consists of titanomagnetites with variable titanium content and therefore different Curie points (Tc). Titanium-rich titanomagnetites with an average titanium content between 20.42 and 25.23% show Curie points of 50–65 °C. In contrast, middle- and poor-Ti titanomagnetites have Curie points between 335 and 480 °C. Titanomagnetites with Tc = 455–515 °C were thermally unstable during heating in argon, while titanomagnetites with a Tc between 335 and 480 °C display thermal instability during heating in air. The magnetic properties of tephra depend on the granulometry. The values of coercive force (Bc), remanent coercive force (Bcr) and the ratio of saturation remanence/saturation magnetization (Jrs/Js) show a directed increase with decreasing grain size, while Bcr/Bc decreases, indicating a different domain state of the magnetic particles in the grain size fractions. The values of magnetic susceptibility (MS) and Js decrease with decreasing size of the fractions for the most tephra samples. The Curie temperatures of magnetic minerals do not depend on the size of the granulometric fractions. Various magnetic mineralogical and geochemical characteristics of some interlayers of tephra allow its use as stratigraphic markers.