RARE-EARTH ELEMENTS IN SUBMARINE HYDROTHERMAL FLUIDS AND PLUMES FROM THE MID-ATLANTIC RIDGE

Rare earth element (REE) measurements were carried out on samples from black- and white-smoker vents from the TAG and Snakepit sites at 26-d egrees and 23-degrees-N on the Mid-Atlantic Ridge. Fluids are substant ially enriched in REEs over seawater, by factors of 10(2) in light-REE s, 10(3) in Eu, and 10(1) in heavy-REEs. White-smoker REE patterns app ear to reflect the effects of shallow subsurface flow. Samples collect ed from within 0. 5 m above the throats of vents (up to approximately 10 times dilution) indicate that the REEs behave in a conservative fas hion with no evidence of removal at this stage of plume evolution. Hig her in the buoyant plume (40-100 m above the vent orifice) where entra inment ratios of seawater to vent fluids are approximately 100-700, di ssolved REEs fall below the dissolved ambient seawater levels (e.g. se awater: Nd = 21.4 pmol kg-1, Ce = 5.44 pmol kg-1, Eu = 1.06 pmol kg-1, Er = 5.47 pmol kg-1; plume waters: Nd = 1.22 pmol kg-1, Ce = 1.12 pmo l kg-1, Eu = 0.35 pmol kg-1, Er = 0.45 pmol kg-1). The dissolved REE p ool shows a net shortfall of 90-98% but the total REEs fall on conserv ative 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 tr end in the REE/Fe ratios of the particles indicate that in addition to initial coprecipitation and uptake, scavenging of REEs must occur dur ing 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.