The dynamics of rare earth elements in soil–plant systems in southern Brazil: a case study of carbonatite-bearing areas and Baccharis trimera

Abstract

Carbonatite outcrops represent significant global sources of rare earth elements (REEs). However, their influence on surrounding soils, vegetation, and REE biogeochemical cycling remains insufficiently understood, particularly in Brazilian carbonatite-bearing systems. Hence, this study investigated the distribution and transfer of REEs in soils and Baccharis trimera near two carbonatite bodies in Caçapava do Sul (Rio Grande do Sul State, southernmost Brazil). Eighty-three samples, including soil, roots, and leaves, were analyzed to determine REE concentrations. Soil analysis revealed significant enrichment in REEs, predominantly comprising light REEs. The soil preserved the geochemical signature of the parent carbonatites, including positive Eu anomalies and no significant Ce anomalies. Monazite, fluorapatite, and pyrochlore were identified as the primary REE-bearing phases. Soil samples from areas closer to the outcrops exhibited significantly higher REE concentrations, whereas more distant samples showed lower concentrations, indicating lithological control. Roots accumulated substantially higher REE levels than the leaves, showing a low bioaccumulation factor and restricted translocation factor, acting as a biological barrier. Strong positive correlations among rock, soil, and plant compartments were observed, confirming that carbonatite lithology directly controls REE distribution across the system. These findings demonstrate the potential of soil geochemistry as a proxy for carbonatite extension and highlight the role of B. trimera roots as sinks, preserving the lithological signature of REE-rich systems.