Tracking and Evaluating the Concentrations of Natural Radioactivity According to Chemical Composition in the Precambrian and Mesozoic Granitic Rocks in the Jangsu-gun Area, Central Southwestern South Korea

Sung Won Kim,Uk Hwan Byun,Saro Lee,Weon-Seo Kee,Byung Choon Lee

doi:10.3390/min11070684

Sung Won Kim, Uk Hwan Byun + Show 3 more

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https://doi.org/10.3390/min11070684

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Journal: Minerals	Publication Date: Jun 25, 2021
Citations: 2	License type: CC BY 4.0

Affiliation: Korea Institute of Geoscience and Mineral Resources

Abstract

The Jangsu-gun area in the central Southwestern South Korea consists of a well-preserved Middle Paleoproterozoic gneissic basement, as well as the Late Triassic and Early Jurassic granitic rocks. Here, we present the detailed zircon U-Pb age data and whole-rock chemical compositions, including radioactive elements (e.g., U and Th) and activity concentrations of 226Ra, 232Th and 40K for the Middle Paleoproterozoic gneisses, and Late Triassic and Early Jurassic granitic rocks of the Jangsu-gun area. The Middle Paleoproterozoic gneissic basement, and the Late Triassic and Early Jurassic granitic rocks have ages of ca. 1988 Ma and 1824 Ma, 230 Ma and 187–189 Ma, respectively. Geochemically, the Middle Paleoproterozoic orthogneiss, Late Triassic granites and Early Jurassic granitic rocks show typical arc-related metaluminous to weakly peraluminous fractionated granite features with ASI (aluminum saturation index) values of 0.92 to 1.40. The mean values of U (ppm) and Th (ppm) of the Middle Paleoproterozoic orthogneisses (6.4 and 20.5, respectively), Late Triassic granites (1.5 and 10.9), and Early Jurassic granites (3.5 and 16.5) were similar to those (5 and 15) of the granitic rocks in the Earth’s crust. The mean 226Ra (Bq/kg), 232Th (Bq/kg), and 40K (Bq/kg) activity concentrations and radioactivity concentration index (RCI) are 62, 71, 1,214 and 0.96 for the Middle Paleoproterozoic orthogneisses; 16, 39, 1,614 and 0.78 for the Late Triassic granites; and 56, 70, 1031 and 0.88 for the Early Jurassic granitic rocks, respectively. The U, Th, 226Ra, 232Th, 40K, and RCI of the Middle Paleoproterozoic biotite paragneisses are similar to those of the Middle Paleoproterozoic orthogneisses. The trend of 226Ra, 232Th, and 40K activity concentrations, and the composition of U and Th from the Precambrian and Mesozoic rocks in the Jangsu-gun area indicates that monazite is the main accessory mineral controlling the concentration of natural radioactivity. Based on a detailed examination of the natural radioactivity in the rocks of the Jangsu-gun area, the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic rocks show average high mean RCI values of 0.88−0.96, such that 32% of the rocks exceeded the recommended value of one in the guidelines for the RCI in South Korea. Especially, the RCI is closely related to the radon levels in the rocks. As a result, the Jangsu-gun area in South Korea is a relatively high radiological risk area, which exhibits higher indoor radon levels in the residences, compared with residences in the other areas in South Korea.

Highlights

To understand natural radioactivity from the granitic rocks in the Jangsu-gun area, we report the concentrations of radioactive elements, such as U, Th, and K, with the activity concentrations of
We report the U-Pb zircon age and geochemical data of the Middle Paleoproterozoic, Late Triassic and Early Jurassic granitic rocks, to clarify the relationships between the concentration trends of natural radioactivity and rock chemical composition
The radiation concentration index (RCI) values for the Middle Paleoproterozoic gneisses and the Early Jurassic granitic rocks in the Jangsu area indicate that 32% of the rocks exceed the RCI guideline value of one for natural stone construction materials in South Korea, and that 58% of the rocks have a high RCI value of 0.8 or more (Table S3)

Summary

Introduction

All minerals and raw materials in the rocks present on Earth commonly contain natural radioactive elements with radionuclides, but natural radioactivity exposures to creativecommons.org/licenses/by/ 4.0/). The natural radioactivity in some rocks can cause high radiation exposure [1,2,3,4,5,6,7]. These rocks have been referred to as naturally occurring radioactive materials (NORMs). Among the NORMs, radionuclides of the 238 U and 232 Th decay series, and 40 K, which potentially occur in the environment, are of the greatest interest. In South Korea, NORMs, such as U and Rn in soil or groundwater, have been investigated by the government for more than 20 years, and their origins have been closely connected with geological factors, such as rock chemical composition, deformation characteristics, and others [2,3,4,5,6,7]

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