Differential behavior of metal sulfides in hydrothermal plumes and diffuse flows

Hydrothermal vents act as a source of reduced metals to the global ocean. Rapid transformations in metal speciation during mixing of hydrothermal fluids with ambient seawater can determine the long-distance transport potential of these elements. Here, extensive sampling of hydrothermal plumes within <2 m of the vent orifice at the East Pacific Rise 9° 50’ N vent field reveals precipitation of metal sulfide phases that vary in size and composition, as characterized by scanning and transmission electron microscopy coupled to energy dispersive X-ray spectroscopy. Despite co-precipitation in phases such as chalcopyrite, iron speciation and transport are de-coupled from that of copper and zinc. Specifically, we observe ~20-50 nm diameter nanoparticulate Zn and Cu sulfide phases that are near-quantitatively removed by filtration through a 0.2 μm filter due to aggregate formation. Whether their aggregation is a filtration artifact or part of a natural particle aggregation and growth process is unclear. In contrast, iron sulfides captured on a 0.2 μm filter typically occur as larger particles up to tens of microns in diameter. Few small iron sulfide nanoparticles (20-100 nm diameter) are captured on the filter. Correspondingly, between 14 to 99 % of the total Fe is measured in the <0.2 μm fraction by ICP-MS (n=42). Determination of nitric acid-soluble Fe in that <0.2 μm fraction analyzed via the spectrophotometric ferrozine method shows that this fraction is not entirely dissolved Fe; rather, up to 10% of the <0.2 μm fraction is nanoparticulate pyrite. To explain the observed size and reactivity differences, we compare nucleation and growth parameters for these different sulfide phases and develop a framework for evaluating metal transport potential at vent sites.