摘要：

Rare earth element (REE) measurements were carried out on samples from black- and white-smoker vents from the TAG and Snakepit sites at 26° and 23°N on the Mid-Atlantic Ridge. Fluids are substantially enriched in REEs over seawater, by factors of 102 in light-REEs, 103 in Eu, and 101 in heavy-REEs. White-smoker REE patterns appear to reflect the effects of shallow subsurface flow. Samples collected from within 0.5 m above the throats of vents (up to 10 times dilution) indicate that the REEs behave in a conservative fashion with no evidence of removal at this stage of plume evolution. Higher in the buoyant plume (40–100 m above the vent orifice) where entrainment ratios of seawater to vent fluids are 100–700, dissolved REEs fall below the dissolved ambient seawater levels (e.g. seawater: Nd = 21.4 pmol kg1, Ce = 5.44 pmol kg, Eu = 1.06 pmol kg1, Er = 5.47 pmol kg1; plume waters: Nd = 1.22 pmol kg1, Ce = 1.12 pmol kg1, Eu = 0.35 pmol kg1, Er = 0.45 pmol kg1). The dissolved REE pool shows a net shortfall of 90–98% but the total REEs fall on conservative mixing lines because of REE uptake by plume particulates. REE/Fe ratios in buoyant plume particles are consistent with a kinetic model for Fe2+ oxidation and coprecipitation of REEs with Fe oxides. The trend in the REE/Fe ratios of the particles indicate that in addition to initial coprecipitation and uptake, scavenging of REEs must occur during dispersion of the particles through the neutral plume. The results of the study demonstrate that scavenging processes, by precipitating Fe-oxyhydroxides, eliminate the impact on seawater of the enrichments of REEs from hydrothermal fluids such that the seawater experiences a net depletion of REEs as a consequence of hydrothermal activity.

展开