Trace Elements and Rare Earth Geochemistry of Clay -Rich Soil from Orin Ekiti, Southwestern Nigeria: Implications on Source Rocks and Paleo-Climatic Conditions

摘要

Ten fresh rocks and twelve clay-rich soil samples were geochemically analyzed using the Inductively coupled plasma -mass spectrometry analytical method at the Central laboratory of the University of Stellenbosch, South Africa. The analyses were carried out to determine the major elements of the rocks as well as the trace and rare earth elements distribution of the soil samples with a view to decipher the source rocks and depositional paleo-redox conditions. The geochemical composition of the surrounding rocks revealed minimum value of 40.13% SiO_2 occuring in charnockite and maximum value of 70.2% SiO_2 in granite. Most of the rocks in the study area fell into felsic class with average SiO_2 value>65%. The Al_2O_3 content is relatively high (12.96 - 21.26 Wt %), reflecting a calc alkaline affinity just as CaO contents ranged from 1.71 to 12.38 Wt %. All estimated weathering indices indicated low degree of weathering of the source rock materials and arid and semi -arid climatic conditions of deposition. Furthermore, the results of the soil samples showed that Ba, Sc, V, Zn and Sr elements are highly enriched than the post Australian average shale (PAAS) and upper continental crust (UCC) respectively. Ratios of Ba/Co (1070.17), La/Th (12.5), and Th/Co (0.98) are significantly higher than the PAAS, thus indicating that the sediments are derived from weathered felsic granitic sources. The data also indicate that the Orin Ekiti clay rich soil has a low LREE/HREE ratio (3.78 to 9.66) in relation to PAAS with a positive Eu giving features similar to a mafic character. The chondrite normalized multi-element diagram showed varied patterns that indicated a slight enrichment of LREE and Pr with a positive European (Eu) implying that samples originated from heterogeneous source rocks. The PAAS normalized REE pattern, showed a pronounced positive Pr, Eu and a negative Nd while the normalized (Gd/Yb)n values presented a range of between 2.36 and 8.86 thus falling within the Archean crust. Geochemical parameters such as Ni/Co, V/Cr and Cu/Zn showed that the clay rich-soil was deposited in well oxygenated environments (oxic condition) while U/Th ratio indicated deposition of sediments under suboxic and dysoxic redox conditions. The clay rich soils of Orin-Ekiti have heterogeneous source rocks that have undergone low to moderate weathering while the soils formed under oxic, suboxic and dysoxic redox conditions.