Controls of fluid chemistry and complexation on rare-earth element contents of anhydrite from the Pacmanus subseafloor hydrothermal system, Manus Basin, Papua New Guinea

摘要

Ocean Drilling Program (ODP) leg 193 successfully drilled four deep holes (126 to 386 m) into basement underlying the active dacite-hosted Pacmanus hydrothermal field in the eastern Manus Basin. Anhydrite is abundant in the drill core material, filling veins and vesicles, cementing breccias, and occasionally replacing igneous material. We report rare-earth element (REE) contents of anhydrite from a site of diffuse venting (Site 1188) which show extreme variability, in terms of both absolute concentrations (e.g., 0.08-28.3 ppm Nd) and pattern shape (La_N/ Sm_N = 0.08-3.78, Sm_N/Yb_N = 0.48-23.1, Eu/Eu* = 0.59-6.1). The range of REE patterns in anhydrite includes enrichments in the middle and heavy REEs and variable Eu anomalies. The patterns differ markedly from those of anhydrite recovered during ODP Leg 158 from the TAG hydrothermal system at the Mid-Atlantic Ridge which display uniform LREE-enriched patterns with positive Eu anomalies, very similar to TAG vent fluid patterns. As the system is active, the host-rock composition is uniform, and the anhydrite veins appear to relate to the same hydrothermal stage, we can rule out predominant host-rock and transport control. Instead, we propose that the variation in REE content reflects waxing and waning input of magmatic volatiles (HF, SO_2) 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 approx 3.5, T = 250-300 deg 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.