METALLIFEROUS SEDIMENTS AND THE SCAVENGING RESIDENCE TIME OF ND NEAR HYDROTHERMAL VENTS

The isotopic composition of Nd is uniform in metalliferous sediments formed at distances varying from > 1000 km to within 10 km of the East Pacific Rise (EPR) palaeoridge. These data indicate that hydrothermal vent fluids, despite having concentrations more than 500 times greater, have no effect on the Nd isotopic composition of seawater. This implicates efficient scavenging of hydrothermal Nd by particulates, resulting in extremely short residence times (< 1 year) close to the hydrothermal vents. Therefore Nd isotopic studies of ancient seawater precipitates, particularly metalliferous sediments, cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates (or residence times) relative to those of modern oceans.