Geoquímica de elementos terras raras em perfis de solo do Arquipélago de Fernando de Noronha

Abstract

The South Atlantic Oceanic Volcanic Islands present a pedogenetic singularity due to their wide variety of soil parent materials and their geographic isolation. Several studies have been carried out in the Fernando de Noronha archipelago, but little is known about the natural contents of rare earth elements (REEs). In this way, this work has as objective to evaluate the influence of mineralogy and geochemistry of different volcanic rocks of Fernando de Noronha on the natural contents, mobility and fractionation of REEs in nine soil profiles of the homonymous main island and Rata island. The mineralogical composition of the rocks was made by the analysis of petrographic slides and analyzed qualitatively and semiquantificatively by scanning electron microscope (SEM) and X-ray dispersive energy (EDS) spectrometer. The samples of rocks and soils were subjected to total digestion for metals and the REEs were determined by optical emission (ICP-OES) and normalized with respect to the rock of origin of each profile. The major elements of the samples were determined by wavelength dispersion X-ray fluorescence spectrometry (XFR), these concentrations were used as the basis for the calculation of the indexes of weathering, chemical alteration (CIA) and mafic index (MIA). The enrichment or depletion of REEs and major elements in the soil profiles were analyzed from the fractionation between light and heavy REEs. Additionally, Ce and Eu anomalies were calculated. In addition to the calculations of the enrichment factors (EF) and mass geochemical balances of the REEs and larger elements to observe geochemical signatures. The results showed that ultrabasic rocks originated soils with higher concentrations of REEs due to their richness in mafic minerals and accessories. Monazite and clinopyroxene are the main sources of REEs in rock samples. The concentration of REEs in rocks and soil profiles decreased in the following order: basanites / ankaratritos > ankaratritos > phonoliths > marine sediments. Despite the strong chemical weathering, soils developed from different lithologies of the Fernando de Noronha archipelago presented different geochemical signatures of REEs. The relationship ΣLREE / ΣHREE showed that the processes of weathering had a significant impact on the fractionation of REE in the soil profiles. In general, typically kaolinite soils have been developed. Thus, the greater fractionation of LREE / HREE was mainly associated to the abundance of kaolinites, which preferentially adsorb LREEs. In general, there was a slight enrichment of LREEs to the detriment of HREEs and a negative anomaly of Ce and Eu positive. Fe2O3, P2O5, TiO2 and higher REE concentrations were found in soils developed from ultrabasic rocks and intermediate and sedimentary rocks. Differences in the composition of primary minerals between ultrabasic and intermediate rocks, as well as the degree of weathering resulted in different mobilization of REEs during weathering. Thus, the mineralogical composition of the source material is a factor for understanding the mobility of the REE during weathering.