Pedogenic fractionation of rare earth elements in chromium- and nickel-enriched ultramafic soils

The geochemical behaviors of chromium (Cr) and nickel (Ni) associated with rare earth elements (REEs) have received limited attention in ultramafic soils. This study hypothesized that ultramafic soils originating from distinct ophiolite complexes exhibit varying amounts of Cr and Ni, with REEs fractionation serving as a fingerprint in these soils. A total of 39 horizon samples from eight pedons, including Entisols, Inceptisols and Alfisols, were collected at different landscape positions from three ophiolite complexes in eastern Taiwan. The total concentrations of major elements, Cr, Ni and REEs were determined. Linear correlation and principal component analysis (PCA) were then used to assess variations in the signatures of REEs corresponding to soil properties, Cr, and Ni. The soils from different ophiolite complexes exhibited various Cr and Ni abundance, and signatures of REEs. REEs remained immobile as their concentrations increased during pedogenesis. The ratio between light REEs (LREEs) and heavy REEs (HREEs) of the soils indicated an enrichment of LREEs compared to HREEs. A common minerals origin of REEs and Cr existed. Thus, REEs and Cr underwent similar geochemical processes in the soils. Additionally, Ce anomaly (δCe) increased along with REEs, Cr, and Ni, corresponding to the formation of secondary iron oxides. Differentiation in ΣREEs, Cr, Ni, ΣLREEs/ΣHREEs, (La/Sm)_N, (La/Yb)_N, (Gd/Yb)_N, and δCe by the PCA can be used to separate the soils from various ophiolite complexes, identifying the origins of these soils in Taiwan. However, further validation with additional data from diverse pedoclimate regions is required for future studies.