Trace and rare earth element geochemistry of the black and grey shales of the Calabar Flank, Southeastern Nigeria: const
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ORIGINAL ARTICLE
Trace and rare earth element geochemistry of the black and grey shales of the Calabar Flank, Southeastern Nigeria: constraints on the depositional environment and the degree of metal enrichment Christopher Iorfa Adamu1 • Benjamin Odey Omang1 • Oluwaseye Peter Oyetade1 Otu Johnson2 • Therese Ntonzi Nganje1
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Received: 18 February 2020 / Revised: 28 July 2020 / Accepted: 3 September 2020 Ó Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract This study focuses on the trace and rare earth elements (REE) geochemistry of the Nkporo and Ekenkpon Shales of the Calabar Flank. The main aim is to infer their depositional environment and the degree of their metal enrichment. The shale samples were analyzed using inductively coupled plasma mass spectrometry. The results indicated that the mean concentrations of K, Na, and Fe in Nkporo and Ekenkpon Shales are 1.45, 0.4, and 4.17 wt%, and 1.11, 0.44, and 5.42 wt%; respectively. The Nkporo Shale is enriched with the following trace elements; P [ Mn [ Sr [ Ba [ Zn [ Ce [ Rb [ Zr [ V[Cr [ Ni and depleted in the following trace elements; Ta [ Ge [ Sb [ Bi [ Cd [ Ag [ Te [ In [ Hg. While the Ekenkpon Shale is enriched with the following trace elements; P [ Mn [ Ba [ Sr [ V[Ce [ Zr [ Rb [ Cr [ Zn [ Ni and depleted in; Sb [ Ge [ Bi [ Ag [ Ce [ Te [ In [ Hg. The concentration of redox-sensitive elements such as V, Ni, Mo, U, Cu, Cr, Re, Cd, Sb, Ti, Mn, and their ratio V/Mo and U/Mo in the black and grey shale samples show different patterns. The REE obtained from the Nkporo and Ekenkpon Shales were PAAS normalized. The Nkporo Shale showed a slightly flat light rare-earth element (LREE), middle rare-earth element (MREE), and heavy rare earth element (HREE) pattern enrichment. Ce/Ce* ranges from 0.95 to 1.09 in Nkporo Shale and 0.67 to 1.40 in Ekenkpon Shale. The Ekenkpon Shale showed a slight LREE, MREE enrichment, and depleted HREE patterns. & Oluwaseye Peter Oyetade [email protected]; [email protected] 1
Department of Geology, Faculty of Physical Science, University of Calabar, Calabar, Cross River State, Nigeria
2
Department of Geology, Faculty of Science, University of Benin, Benin, Edo State, Nigeria
The Mn contents and U/Mo ratio in Nkporo and Ekenkpon Shales suggests a poor oxygen transitional environment. The V/Mo and V/(V ? Ni) ratios indicated that the Nkporo shales were deposited in an anoxic to suboxic conditions and Ekenkpon shales were also deposited under an anoxic to suboxic conditions. The V/Ni ratio indicated that the organic matter in the Nkporo shale is terrigenous while that of the Ekenkpon shales are both terrigenous and marine in origin. Keywords Rare-earth elements (REE) Trace elements Calabar Flank Suboxic Anoxic
1 Introduction In most hydrocarbon producing clastic sedimentary basins of the world, shales are referred to as the source rock, because of their hydrocarbon generating potential. Shales contain organic matter (kerogen). This kerogen (type
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