Geochemistry of Rare Earth Elements in Rocks and Soils along a Cretaceous Volcano-Sedimentary Basin in Northeastern Brazil

Volcanic rocks are widely distributed throughout the Earth's continental crust and are key factors in soil formation that play a fundamental role in the global geochemical pattern of rare earth elements (REEs) in the pedosphere. This research was carried out to better understand the geochemistry of REEs in rocks (ignimbrite, rhyolite, conglomerate, trachyandesite, and basalt) and their respective soils along a Cretaceous volcano-sedimentary basin in the humid tropical environment of Northeastern Brazil. Despite the high rate of chemical weathering, the soils derived from volcanic rocks exhibited distinctive geochemical signatures of REEs. The data reveal complex interplay between the composition of the parent material and the soil properties that control the REE geochemistry in soils. The mean REE concentrations within the lithosequence decreased in the order (mg kg−1): rhyolite (233.4), trachyandesite (182.6), ignimbrite (172.2), and basalt (83.2). Calculations of the LaN/YbN ratio showed that no fractionation occurred in ignimbrite (0.3) and rhyolite (0.4). However, slight fractionation was observed in trachyandesite (1.1) and basalt (2.1). Feldspar, zircon, biotite, apatite, ilmenite, magnetite, and olivine are the main REE sources in the soils. The mean REE concentrations in the soils decreased in the following order (mg kg−1): soil under trachyandesite (231.9), soil under basalt (182.0), soil under rhyolite (106.9), soil under conglomerate (99.3), and soil under ignimbrite (51.8). These findings illustrate that pedogenesis has an important impact on REE distribution in soils. This research highlights the effect of the volcano-sedimentary lithosequence on REE geochemistry in soils in a humid tropical climate.