Yttrium and rare earth elements in fluids from various deep-sea hydrothermal systems

摘要

Rare earth element (REE) and yttrium (Y) concentrations were measured in fluids collected from deep-sea hydrothermal systems including the Mid-Atlantic Ridge (MAR), i.e., Menez Gwen, Lucky Strike, TAG, and Snakepit; the East Pacific Rise (EPR), i.e., 13 deg N and 17-19 deg S; and the Lau (Vai Lili) and Manus (Vienna Woods, PacManus, Desmos) Back-Arc Basins (BAB) in the South-West Pacific. In most fluids, Y is trivalent and behaves like Ho. Chondrite normalized Y-REE (Y-REE_N) concentrations of fluids from MAR, EPR, and two BAB sites, i.e., Vai Lili and Vienna Woods, showed common patterns with LREE enrichment and positive Eu anomalies. REE analysis of plagioclase collected at Lucky Strike strengthens the idea that fluid REE contents, are controlled by plagioclase phenocrysts. Other processes, however, such as REE complexation by ligands (Cl~-, F- SO_4~(2-)), secondary phase precipitation, and phase separation modify REE distributions in deep-sea hydrothermal fluids. REE speciation calculations suggest that aqueous REE are mainly complexed by Cl~- ions in hot acidic fluids from deep-sea hydrothermal systems. REE concentrations in the fluid phases are, therefore, influenced by temperature, pH, and duration of rock-fluid interaction. Unusual Y-REE_N patterns found in the PacManus fluids are characterized by depleted LREE and a positive Eu anomaly. The Demos fluid sample shows a flat Y-REE_N pattern, which increases regularly from LREE to HREE with no Eu anomaly. These Manus Basin fluids also have an unusual major element chemistry with relatively high Mg, SO_4, H_2S, and F contents, which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation. REE distribution in PacManus fluids may stem from a subseafloor barite precipitation and the REE in Demos fluids are likely influenced by the presence of sulfate ions.