Rare Earth elements in rocks and soil profiles of a tropical volcanic archipelago in the Southern Atlantic

Abstract

Rare earth elements (REEs) concentration and signature in soils are influenced by parent material and pedogenesis, but little information is available on the potential of REEs to trace pedogenetic processes in tropical volcanic soils. The archipelago of Fernando de Noronha, Brazil, represents a mostly pristine environment with a diversity of parent materials varying from ultrabasic igneous to sedimentary rocks. Here, we collected nine soil profiles to study the parent materials' petrography and the chemistry of soils from the archipelago to interpret the signature and fractionation of REEs in soils along a volcanic lithosequence. Besides, soil guideline values (SGVs) for REEs were established in the soils of the archipelago. REEs in the rocks were found mainly in mafic and accessory minerals such as apatite, allanite, monazite, nepheline, clinopyroxene, and olivine. Owing to the higher amounts of mafic minerals in the ultrabasic rocks, the total REE content in the parent materials followed the order: basanite / ankaratrite > ankaratrite > phonolite > sedimentary deposits of marine origin. Likewise, the REE mean contents in soil profiles diminished in the order: Cambisols (339 mg kg−1) > Vertisols (227 mg kg−1) > Arenosol (68 mg kg−1). The SGVs for REE (mg kg−1) were Ce (165.7), La (83.1), Nd (55.0), Pr (17.0), Sm (11.2), Dy (9.9), Gd (4.3), Ho (2.9), Er (2.4), Tb (1.8), Yb (1.6), and Lu (0.6). Although the composition of parent materials was crucial for the differentiation of REEs, pedogenesis altered the accumulation and distribution of REEs in soils substantially. Thus, REE signatures of parent rocks were maintained in the soils, but concentration and stability of secondary phosphate minerals and iron oxides controlled the REE fractionation in the soil profiles.