Rare earth elements in iron oxy-hydroxide rich sediments from the Marabasco River-Estuary System (pacific coast of Mexico). REE affinity with iron and aluminium

Rare earth elements (REE from La to Lu), Al and Fe were analysed in 35 surface sediment samples from the Marabasco River-Estuary system, located in the active tectonic zone of Cocos Plate near the active Colima Volcano. The Peña Colorada iron mine is found in the Marabasco watershed and the river sediments are enriched by iron oxy-hydroxides. The average REE content is 86.9 ± 26.8 mg·kg^{- 1} and 95.5 ± 50.3 mg·kg^{- 1} for river and estuarine sediments respectively, which is low in comparison to other tropical coastal systems. The REE light/heavy content ratios (L/H) of Marabasco sediments are also low with averages of 5.2 ± 0.8 (river) and 5.7 ± 0.9 (estuary) due to the minor presence of LREE. The REE signature of the sediment by normalisation with chondrite indicated a slight negative anomaly of Europium and the absence of a Cerium anomaly. REE concentrations in the Marabasco sediments show a strong correlation with Al and Fe, with LREE having a greater affinity for Fe and HREE for Al. This antagonism is REE-ordered by their atomic number sequence in the river sediment case. This has not been previously observed in the environment and the presence of magnetite and the iron oxy-hydroxide enrichment may enhance it. This did not occur in the estuary of the Marabasco mine region where Al and Fe are both well correlated.