Radionuclides in rocks of southern part of Mtskheta-Mtianeti region (Georgia)

Abstract

This study investigates distribution of natural and technogenic radionuclides in rock samples in the southern part of the Mtskheta-Mtianeti region, located in the so-called Transcaucasian intermountain area (II) and in the Lesser Caucasus fold system (III). Several geotectonic units are identified here: the eastern immersion zone or Georgian Block (II3), the molassic subzone of Kartli (II31), the central (axial) subzone (III13), and the southern subzone (III14) of the Adjara-Trialeti folded zone (III1). The radioactivity of rocks in this region has not been investigated. Twenty samples of rocks of various types, including sedimentary and metamorphic, were investigated with the gamma-spectroscopic method. Up to 21 naturally occurring radionuclides and one technogenic radionuclide were identified in samples. Average activity concentration of Th-232 family radionuclides varied from 4.5 to 67.0 Bq/kg, U-238 family: from 4.5 to 93.5 Bq/kg, and U-235 family: from 0.22 to 4.3 Bq/kg. The highest activity concentration was observed for K-40 (maximal value of 746 Bq/kg). Activity of technogenic radionuclide Cs-137 varied from 0.12 to 6.3 Bq/kg. There are some marked features in radionuclide distribution depending, in particular, on rock type, tectonic zone, and age. Some radionuclide activity ratios have been considered: U-238/U-235, U-238/Th-232, Ra-226/U-238, and Pb-210/Ra-226. These ratios allowed estimation of the system's condition (closed or opened) and information about the character of relevant geochemical processes. Comparison was carried out with existing data in the literature.