Rare-earth element concentration of anhydrite vein separated from ODP Site 1188 (Table 2)

Instead, we propose that the variation in REE content reflects waxing and waning input of magmatic volatiles (HF, SO2) and variable complexation of REEs in the fluids. REE speciation calculations suggest that increased fluoride and possibly sulfate concentrations at Pacmanus may affect REE complexation in fluids, whereas at TAG only chloride and hydroxide complexes play a significant role. The majority of the anhydrites do not show positive Eu anomalies, suggesting that the fluids were more oxidizing than in typical mid-ocean ridge hydrothermal systems.

We use other hydrothermal fluids from the Manus Basin (Vienna Woods and Desmos), which bracket the Pacmanus fluids in terms of acidity and ligand concentrations, to examine the dependence of REE complexation on fluid composition. Geochemical modeling reveals that under the prevailing conditions at Pacmanus (pH=3.5, T=250–300 °C), Eu oxidation state and the relative importance of fluoride versus chloride complexing are very sensitive to small variations in oxygen fugacity, temperature, and pH.

Patterns with extreme mid-REE enrichment may reflect speciation effects (free-ion abundance) coupled with crystal chemical control. We conclude that the great variability in REE concentrations and pattern shape is likely due to variable fluid composition and REE complexation in the fluids.