Sulfate minerals control dissolved rare earth element flux and Nd isotope signature of buoyant hydrothermal plume (EMSO-Azores, 37°N Mid-Atlantic Ridge)
Résumé
While hydrothermal vents are now thought to be a major source of dissolved iron to the oceans, they have always been considered to be a sink for the dissolved rare-earth elements (DREEs). However, true dissolved REE observations in hydrothermal plumes are still lacking. Here we report for the first time the DREE concentrations and neodymium isotopic compositions (DεNd) of buoyant hydrothermal fluids at Lucky Strike (Mid-Atlantic Ridge). We find that 27 to 62% of total hydrothermal DREEs are rapidly scavenged by anhydrite precipitation at the onset of buoyant plume formation. After this initial loss, all DREEs behave quasi-conservatively within the buoyant plume. Dissolved phase εNd (DεNd) in the evolving plume are identical to black smoker DεNd of +9.0 and contrast radically with DεNd of the local deep water mass at −12.0. Plume DεNd as low as +6.6 may be reconciled by dissolution of newly formed barite in the local environment and carrying seawater DεNd signature. We find, based on the first plume DREE observations, that hydrothermal plumes are in fact a source of DREE to the North Atlantic Deep Water. Precipitation/dissolution processes of hydrothermally-derived minerals, i.e. sulfates in the buoyant plume and Fe oxy-hydroxide in the non-buoyant plume, will likely affect the fate of other trace metals and their isotopic composition.
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