Rare Earth Element (REE) Geochemistry During Red Soil Formation in Southern China

Abstract

The distribution of rare earth elements (REEs) in red soil profiles in southern China was studied in detail. Much higher total REE contents were observed in red soils than in the bedrocks. The highest total REE contents were found in gneiss-, limestone- and granite- derived red soils and the lowest in quartzitic sediments. There were no significant differences in total REE contents between the red soil horizons although soluble REEs accumulated in the lower horizons of the profiles. Amounts varied greatly with parent materials. There was a significant exponential relationship between the total REEs and soluble REEs in red soils. Soluble REEs were also positively correlated to a chemical index of alteration (CIA) values and clay contents, but were negatively correlated with organic matter, pH values and Al2O3/Fe2O3 mole ratios. These results imply that soluble REE contents could be a useful indicator of red soil development. Total REE contents were negatively correlated with SiO2/Al2O3, Al2O3/Fe2O3 mole ratios, but were positively correlated with the R2O3/SiO2 mole ratio, reflecting the fact that REEs are mainly inherited from parent rocks. It was concluded that soluble REEs tended to be leached out from the red soils at an early stage of soil development and that residual forms became enriched at a later stage. The main form of REEs in red soils is as a residual phase associated with primary minerals inherited from the parent rocks, while the non-residual forms are closely related to the laterization process.