Clay minerals control rare earth elements (REE) fractionation in Brazilian mangrove soils.

• Kaolinite transforms into Fe-smectite and Fe-illite in mangrove soils. • REE were not influenced by anthropic activity. • There is a trend of depletion of Heavy REE (HREE)in the normalized patterns. • Terrigenous kaolinite control the fractionation of light REE (LREE) over HREE. • Adsorption of HREE by 2:1 clays reduced the fractionation of LREE over HREE. Mangroves are intertidal tropical ecosystems influenced by marine and terrigenous input. They are important sinks of trace elements, like Rare Earth Elements (REE), which are recognized as excellent indicators of geochemical processes in estuaries. REE data of bulk soil samples from eight mangroves located along the Brazilian coast were correlated with quantitative clay mineral data based on XRD full-profile modelling. Kaolinite has been found to transform to Fe-illite in these mangroves. The clay assemblage was dominated by interstratified phases, including kaolinite-smectite (kaolinite and smectite-rich), illite–smectite and other illitic phases. REE concentrations in bulk soils were LREE > MREE > HREE (where L, M and H indicate light, medium and heavy). Multivariate statistical approach (PCA analysis) using clay mineral concentrations and proxies for relative REE concentrations ([La/Gd] N , [La/Yb] N and [Gd/Yb] N ratios) showed (r > 0.75) that fractionation of LREE over HREE was highest in mangroves where kaolinite, fed with the highly weathered sediments form the Neogene Barreiras Group, was most abundant (67–85% of all clay minerals), while REE fractionation decreased as the relative proportion of Fe-rich smectite increased (26–46 % of all clay minerals). Identified processes of REE control are: 1) Level of sediment weathering within the continent, where higher weathering implies higher kaolinite content and higher REE fractionation (LREE > MREE > HREE); 2) selective leaching of LREE from mangrove soils by saline water; 3) preferential adsorption of HREE by Fe-rich smectite and illite neoformed in the mangrove soils. It is possible to conclude that clay minerals exert an important control on REE fractionation in tropical mangroves. [ABSTRACT FROM AUTHOR]