Radioactivity, radiogenic heat production and environmental radiation risk of the Basement Complex rocks of Akungba-Akoko, southwestern Nigeria: insights from in situ gamma-ray spectrometry

Abstract

The Basement Complex rocks of Akungba-Akoko, southwestern Nigeria, typically migmatites (M), biotite gneisses (BGN), granite gneisses (GGN), charnockites (Ch), biotite granites (BG) and pegmatites (P), were assessed using ground gamma-ray spectrometry (GRS). This study aimed at determining rocks’ radionuclides concentration, alterations, radionuclides mobilization, radiogenic heat production (RHP) rate (first report for the area) and environmental radiation risk to humans. The results of this study were integrated with previous data on the petrography and geochemistry of these rocks, for detailed interpretation. The results of average elemental and activity concentrations for Akungba-Akoko rocks, were 2.66%, 3.16 ppm and 13.98 ppm, and 831.35, 39.01 and 56.77 $$Bq{kg}^{-1}$$ for 40K, 238U and 232Th, respectively; with increasing rock order of Ch < M < GGN < BGN < P < BG. High radionuclides concentration in the rocks, with exception of charnockites, were determined by high amount of K-feldspar, plagioclase, biotite and accessory minerals—zircon and monazites. On the other hand, the low radionuclides concentration in the rocks, especially in Ch and M, were aided by the variability of the naturally occurring radioactive elements (NORMs) caused by alterations and mobilization during metamorphism and crystallization processes as suggested by radioelements composite, K/eTh ratio and F-parameter of Efimov analyses. RHP average of 2.03 $$\mu W{m}^{-3}$$ above the crustal average range of 0.8–1.2 $$\mu W{m}^{-3}$$ may have significantly contributed to the heat flux in the studied area; however, the RHP of charnockites are lower than the crustal range. The absorbed dose rate average of 87.98 $$nGy{ h}^{-1}$$ was within the permissible range, and other estimated radiological parameters, namely annual effective doses, hazard indices, gamma activity and activity utilization index, were all far below the permissible limit of 1 $$mSv {y}^{-1}$$ for these rocks. However, the annual gonad dose equivalent (618.874 $$\mu Sv{ y}^{-1}$$ ) and excess life cancer risk (1.511 × 10− 3) were above their permissible limits. Due to the high gamma radiations from the pegmatitic and biotite-rich rocks in the studied area, their high usage should be reduced. Hence, periodic monitoring of the study area is advised.