Geochemistry of REE and yttrium in hydrothermal fluids from the Endeavour segment, Juan de Fuca Ridge

抄録

Rare earth element and yttrium in hydrothermal fluids sampled from six high temperature vents and two diffuse flow sites on the Endeavour segment, Juan de Fuca Ridge by US-CHINA Joint Deep Dive Cruise in Aug. 2005 were analyzed by ICP-MS after being pre-concentrated by 20-fold. According to the species distributions calculated by GWB, the chondrite-normalized REE patterns (REE_CN) of these fluids are characterized by LREE enrichment and positive Eu anomalies, which are mainly controlled by complexing with chloride. The (La/Yb)_CN ratio does not correlate significantly with fluoride. Sulfate content are low in most high temperature fluids except in Gremlin vent fluids. Certain amounts of REE are complexed with sulfate that likely originates from oxidation of magmatic SO₂ and/or H₂S in Gremlin. Sulfates, mostly come from seawater, are also responsible of large percentage of REE complexes in diffuse fluids. It is shown by the character of REY concentrations and the Y/Ho ratios that REE removed by Fe, Mn-oxyhydroxides is negligible in most of high temperature vent fluids. However, the Y/Ho ratios in Gremlin (Y/Ho = 160) and Faulty Towers (Y/Ho = 140) vent fluids are higher than that in other high temperature vent fluids, suggesting REE removal controlled by coprecipitation and scavenging. The negative Ce anomalies in diffuse fluids also result from the scavenging of mineral particles in the oxidizing conditions when mixing with seawater. Compared to ideal mixing fluids calculated by Mg content, two diffuse fluids display LREE-depleted patterns and enhanced positive Eu anomalies. Based on the REE characteristics of anhydrite, the REE distribution patterns of diffuse fluids are probably influenced largely by anhydrite precipitation under the seafloor at diffuse fluids sites. It is interesting to notice that the HREE_CN pattern of Mothra vent fluids is similar to sediments but different from the sediment-starved hydrothermal fluids, suggesting they are likely remolded by the sediments in water-rock interaction zone and/or upflow zone nearby Mothra vent field. The provenance and character of these sediments bears further investigation.