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REE Mineralogy and Resources Chapter 279 163 enriched in LREEs by factors of 50–100, 2000–7000 in Eu, and by factors of 5–30 in HREEs compared with seawater. Rare earth enrichments in seafloor hydrothermal fluids relative to ambient seawater reflect removal from crustal rocks during fluid–rock interaction. It has been suggested that plagioclase dissolution controls the distribution of REEs in submarine hydrothermal fluids because the chondrite-normalized REE compositions of mid-oceanic ridge hydrothermal fluids and plagioclase are similar with prominent positive Eu anomaly (Campbell et al., 1988; Klinkhammer et al., 1994). Laboratory studies, however, suggest that REE compositions of seafloor vent fluids are unrelated to primary rock composition because REE distri- bution patterns in experimental hydrothermal solutions are different from primary REE compositions of the volcanic rocks or individual minerals with which these fluids have reacted (Allen and Seyfried, 2005; Bach and Irber, 1998). These studies suggest that vent fluid REE compositions reflect solubil- ity control during fluid–rock interaction influenced by aqueous REE specia- tion, which in turn is strongly influenced by numerous aspects of fluid chemistry such as pH, temperature, and the availability of ligands. In the backarc spreading centers (eg, Manus Basin), hydrothermal fluids are also enriched in REEs similar to the fluids at mid-oceanic ridges (Fig. 18B). However, the REE compositions of the fluids at the backarc cen- ters are not uniform like those at the mid-oceanic ridges, but are quite various; some enriched in LREEs and some enriched in HREEs (Craddock et al., 2010). In the Manus Basin fluids, aqueous REE compositions are not inherited directly from those of the primary crustal rocks with which hydrothermal fluids interact, but are consistently correlated with differences in fluid pH and ligand (chloride, fluoride, and sulfate) concentrations. For example, hydrothermal fluids from the DESMOS and North Su vents, which are char- acterized by low pH <1.8 (measured at 25°C) acid-sulfate solutions, show higher concentrations of REEs than those of high-temperature black smoker fluids at Vienna Woods and PACMANUS, but are similar to those of acid- sulfate fluids from continental geothermal environments. These fluids show a flat chondrite-normalized REE pattern (Fig. 18C), which is significantly different from the LREE-enriched pattern with positive Eu anomaly at mid-oceanic ridges and other vents in the backarc setting (Craddock et al., 2010). These data suggest that REE compositions from the vent fluids are fractionated from primary crustal rock REE compositions during fluid–rock interaction owing to differences in aqueous REE solubility, and the elevated REE concentration in the acid-sulfate fluids is ascribed to low pH of the fluids that extracted REE efficiently from the host rocks (Craddock et al., 2010). Klinkhammer et al. (1983) and Michard et al. (1983) showed that the pre- cipitation of hydrothermal iron oxyhydroxide particles leads to REE scaveng- ing. Mitra et al. (1994) concluded that more than 95% of REE are removed from the fluids by iron oxide particles, on the basis of the hydrothermal fluids from the Trans-Atlantic Geotraverse vent and Snakepit sites of thePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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