Heat production and seismic velocity in the basement lithology of Ago-Iwoye, Ogun State, Nigeria

Abstract

The nature of the rocks and their geological composition are very essential in the estimation of heat flow and geodynamics since heat production varies in rock types and climate to climate. The main focus of this study is to examine the heat production and seismic velocity associated with the basement lithology of Ago–Iwoye, Ogun State. Three lithological units; undifferentiated schist, migmatite, and quartzite schist were identified across the study area. A total of 30 rock samples were collected, with 10 samples randomly selected from each unit, to evaluate their geochemical (H) and radiogenic (A) heat production potential. The geochemical analysis of heat produced and the seismic velocity ( P V ) were estimated with the concentration of the major oxides (SiO₂, Al₂O₃, CaO, FeO, K₂O, MgO, Na₂O, P₂O₅) which were analytically determined using X-ray Fluorescence Spectrometer. Radiometric analysis was carried out using a gamma-ray spectrometer to assess uranium (U), thorium (Th), and potassium (K) concentrations, therefore enabling the calculation of radiogenic heat. The thermodynamic model was adopted in estimating the geochemical heat produced while the Rybach model was used to compute the radiogenic heat produced. The empirical relations between seismic velocity and heats produced were deduced. Three geochemical proxies; Fe number (Fe*), chemical index of weathering (CIA) and maficity were also examined. The result obtained revealed that the basement rocks of the study area are S-type igneous rocks and peraluminous in nature. The heats produced for H and A are found to exhibit an inverse relationship with seismic velocity across the lithological units. The H-heat was found to range
from 4.20 – 5.63 µWm^-3 while the A- heat generated was in the range of 2.97 – 13 18 µWm^-3. A very strong negative correlation value of -0.99 was established between H and P V . The H-heat and A-heat are found to be comparable with a correlation value of 0.78. SiO2 accounted for good linear variation with A, H, P V and U. A log-linear relationship was established between geochemical heat, radiogenic heat and seismic velocity. The result obtained shows that the log-linear value of both geochemical and radiogenic heat have inverse relation with seismic velocity with r-values of -0.84 and -0.98. The established relation between the heat produced and seismic velocity can therefore be adopted as a first order approximation for geothermal assessment of the basement region of the earth’s crust.