The Mobility of Rare-Earth Elements During Hydrothermal Activity:A Review

The mobility of the rare-earth elements(REE)during hydrothermal activities is increasingly doc-umented.Geological and experimental evidence suggests that REE may be mobile in solutions rich inF~-,Cl~-,HCO~-,CO,HPO, PO,or in combinations of the above ligands,even thoughlittle has been known about which ligand or which combination is most effective in mobilizing REE.The fractionation of REE resulting from hydrothermal activities is inconsistent.One set of field dataimplies the preferential mobility of the light rare-earth elements(LREE),whereas another set of fieldobservations indicates the dominant mobilization of the heavy rare earth elements(HREE),and sometheoretical prediction is contradictory to the field evidence.The Eu anomalies due to hydrothermal ac-tivities are complex and plausible explanation is not available.The existing experimental approachesdealing with REE are not adequate for explanation of REE behaviour in aqueous solutions.Systemat-ic experimental approaches are suggested.