Comportamento biogeoquímico de elementos traços com ênfase nas terras raras no sistema planta-rocha – Lavras do Sul/RS

Abstract

In this work, a NE-SW profile of approximately 6 km was performed to analyse the distribution behavior of the major and trace elements with emphasis on Rare Earths, in seven rock and plant samples (Baccharis trimera - Carqueja), belonging to the same collection point, inserted in the SW portion of the Intrusive Complex of Lavras do Sul. To aid in the characterization of the facies occurring in the region, petrography was used, separating the geological domain into three parts: a central zone in facies granodiorite, intermediate in monzogranite and distal facies of sienogranite. In the chemical study, the concentrations obtained by the ICP-MS and FUS-MS methods were used as parameters in the comparison between rocks (lithochemistry) and vegetal (biochemical) samples. The Rare Earths are concentrated in the CILS following the M type tetrad effect, typified by negative anomalies of Nd, Gd, Ho-Er, being an exception to the phenomena, the W-type effect, present at the sienogranito facies, which is characterized as a advanced phase of fractional crystallization, where the relative enrichment of the mentioned elements occurs by mobilization of later hydrosaline fluids. For the carqueja, the opposite factor was observed, being the predominant type W effect, demonstrating relative enrichment of Nd, Gd, and Ho-Er pair. This has demonstrated that the exogenous processes associated with the biochemical functions of the rhizosphere are concentrating the said elements. Although these differences occur due to the peculiarities mentioned, it is clear that the chemical signature of trace elements is very similar in both cases (rock and plant), suggesting that regardless of the type of substrate, Baccharis trimera may reflect its residence location. This means that carqueja has been shown to be a fair indicator of signature patterns of complex formation in hydrothermal and igneous systems. In the environmental area, the tetrad effect, mainly related to the positive Gd anomalies, can act as an aid in the identification of the free ions nature in the rhizosphere, soil or water, indicating if they are natural from the rock or if they come from anthropogenic actions .