Use of rare earth elements and internal transition elements for characterization of the depositional paleo environment in samples of the experience of the Pimenteiras Formation, Parnaíba Basin, Brazil

Abstract

The quality of the organic matter together with the conditions of the depositional paleoenvironment at the top of the Pimenteiras Formation (Devonian), were geochemically characterized using 11 (eleven) samples of shales collected from meter to meter in outcrop located on the western edge of the Parnaíba Basin, the northeast region of Brazil. The inorganic geochemical characterization was carried out through the analysis of rare earth elements and uranium, and transition elements (Ni, V, Co, and Mo) by ICP OES for Ni and V. For the rest of the elements, the determinations were made by ICP MS. The transition elements allowed to evaluate the origin of the organic matter using the diagram of the concentration ratio between the Ni/Co elements versus the concentration ratio for the V/Ni elements and the depositional paleoenvironment using the V concentration versus Ni concentration diagram. The uranium and molybdenum relationships established the presence or not of sulfides (euxinic or non-euxinic environment). The first group consists of samples from the lower third (base) of the outcrop, which has mixed organic matter, predominantly terrestrial, with the contribution of fresh/brackish water organisms, deposited under paleoenvironmental conditions from oxic to suboxide. The second group consists of samples collected in the upper third (top) of the outcrop containing mixed organic matter with a predominance of marine organic matter and little terrestrial contribution, deposited under anoxic paleoenvironmental conditions. The sum of the concentrations of rare earth elements suggested the existence of two sets of samples containing organic matter from different sources. The organic geochemical characterization was performed using analytical techniques to evaluate the percentage of total organic carbon, total sulfur, Rock-Eval pyrolysis data, and for diagnostic reasons of saturated biomarkers. The shales of the first group poor in total organic carbon (TOC), have the sum of the concentrations of rare earth elements lower than the second group, which are rich in organic matter. According to analyzes of the saturated biomarkers, about the depositional paleoenvironment of the Pimenteiras Formation in the area of that outcrop, the organically poorest shales of the first group were deposited in a transitional depositional paleoenvironment, with a large supply of continental organic matter, while the most enriched in organic matter. organic matter represents a marine flood surface, where the organic material is predominantly marine.