Geochemistry of major and trace elements and Pb-Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan, China

© 2014 Elsevier Ltd. This paper reports a geochemical study on the major and trace elements and Pb-Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe2O3, FeO, MnO, MgO, CaO, Na2O, and P2O5 and of less variable concentrations of SiO2, TiO2, Al2O3, and K2O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO2, FeO, CaO, K2O, Na2O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al2O3, MgO, Fe2O3, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO2, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb-Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite-smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na2O, K2O, P2O5, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO2 and Al2O3). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.