Influência de granitos anorogênicos (Tipo A) na geoquímica de elementos terras raras em solo no semiárido brasileiro

Abstract

The global demands for rare earth elements (REEs) are continuously increasing and, consequently, the accumulation of these elements in the soil and the environment has caused adverse effects on human and environmental health, causing growing concern not only in the scientific community, but throughout society. The geochemistry of REEs in soils remains poorly understood, particularly in semi-arid environments. Studies on the dynamics of REEs in different geological, pedological and climatic contexts are necessary to understand the different biogeochemical behaviors of these elements. However, there are still no studies on the effect of type A granites on the physical, chemical and mineralogical attributes of soils located in different environmental conditions. Thus, the present work aimed to evaluate the influence of the mineralogy of anorogenic granites (A Type) on the mineralogy and geochemistry of rare earth elements in the Brazilian semi-arid region. The soil profile was selected based on the geological map of Pernambuco. A Type granite was analyzed using a petrographic microscope, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Soil morphology was described based on the Soil Description and Collection Manual in the Field and the soil profile classification was according to the Brazilian Soil Classification System. REEs readings were made by optical emission spectrometry. An X-ray diffractometer was used to identify the minerals in the soil fractions. The soil profile derived from type A granite in the semi-arid region was classified as CAMBISSOLO HÁPLICO Ta Eutrófico Típico, defined by the presence of an incipient B diagnostic horizon (Bi), being deep, not stony, not gravelly and not rocky. Based on the fertilizer recommendation manual for the state of Pernambuco, the natural fertility of the soil profile originating from type A granite was considered moderate. A Type granite showed a high proportion of mafic and accessory minerals, mainly biotite, hornblende, allanite, garnet, opaque minerals and apatite. Bastnasite and monazite weathering were the main sources of REEs in the soil. The average total concentration of REEs in the soil profile originating from A type granite was very high (394.2 mg kg-1), being higher than the average of soils derived from other granite types (I and S Types), as well as from average of the soils of Europe, China, Japan, Sweden and the reference soils of Brazil. Soil derived from A type granite showed REE enrichment, with slight fractionation between LREEs and HREEs. The LaN/YbN ratio slightly higher than one also confirmed the slight fractionation between LREEs/HREEs. There was no fractionation between LREEs (LaN/SmN ratio <1). However, there was a slight fractionation among the HREEs (GdN/YbN ratio > 1) due to these metals being more mobile than the LREEs. The positive cerium (Ce) anomaly is explained by the oxidizing condition of the profile, in which Ce3+ transforms into Ce4+, presenting low solubility and a greater tendency to enrichment. The positive europium (Eu) anomaly is explained by the substitution of Eu by strontium (Sr2+) in plagioclase. Therefore, the positive Eu anomaly in the soil reflects the same geochemical signature in its parent material. The mineralogical composition of A type granite, as well as the mineralogy of the soil, had a strong influence on the geochemistry of REEs in the soil. This study provides evidence of the influence of A type granite on the geochemistry of REEs in soil located in the Brazilian semi-arid region.