HYDROTHERMAL SCAVENGING AT THE MID-ATLANTIC RIDGE - MODIFICATION OF TRACE-ELEMENT DISSOLVED FLUXES

We have carried out an ICP-MS study of the distributions of trace elements (Fe, Ca, Mg, Al, Mn, Cu, Zn, V, As, Cr, Co, Pb, Y, Mo, Ag, Sn, Cd and Be) in particulate material collected from the TAG hydrothermal plume (26-degrees-N, Mid-Atlantic Ridge) to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent-fluids are balanced by the precipitation of associated particulate phases. Comparison of particulate trace element distributions with particulate Fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes: (i) co-precipitation from vent-fluids, (ii) scavenging from seawater, and (iii) preferential settling from the plume and oxidative dissolution within the water column. For Y and Be, continuous scavenging from seawater appears to be important, whereas for V, As and Cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume. From a comparison of trace element: Fe ratios in the particles and associated Mid-Atlantic Ridge vent-fluids it appears that for many elements (e.g., Cu, Co, Ag, Pb, Al, Cd, Be and Zn) the flux of dissolved material from vent-fluids is matched closely by the precipitation of hydrothermal particulate phases. Of the various elements studied here, hydrothermal activity only appears to act as a significant source for dissolved Ca and dissolved Mn.