Submarine hydrothermal venting related to volcanism in the Lesser Antilles: Evidence from ferromanganese precipitates

[1] Radiogenic isotope compositions (Sr, Nd, Pb, Hf, and Os) of sediment-hosted seafloor ferromanganese crusts and sediments incrusted with ferromanganese oxyhydroxides from the Lesser Antilles island arc were measured to distinguish between hydrogenous (seawater-derived) and hydrothermal metal sources. The ages of the precipitates range between recent (last few thousand years) and a few 100 kyr as deduced from 10Be and Co concentrations. Evidence from the presence of bladed todorokite and nontronite, together with the major element and REE composition, suggests that a significant proportion of these sediment-hosted precipitates formed at relatively low temperatures from a mixture of seawater and hydrothermal fluids associated with island arc volcanism. The radiogenic isotope compositions of all metals mentioned above, except Pb, show large differences in hydrothermal versus hydrogenous contributions over space and time. In contrast to precipitates of high-temperature fluids which mainly scavenge their REE contents from seawater the crusts of this study show 143Nd/144Nd of up to 0.512817 (ɛNd = +3.5). This is close to the signature of the nearby island arc rocks and far above the expected local seawater ratio of ∼0.51209 (ɛNd = −10.7). These crusts also show high 176Hf/177Hf (up to 0.283102), low 87Sr/86Sr (up to 0.7069), and low 187Os/188Os (up to 0.16) compared with local seawater, as expected from hydrothermal, island-arc-derived metal contributions. In contrast, the Pb isotope signatures of the crusts cannot be explained by mixing between seawater and hydrothermal sources. It is suggested that Pb was either removed from the ascending fluids within the sediment column before they reached seawater or the temperatures were too low to leach significant amounts of Pb from the rocks or sediments. External sources such as Saharan dust, particulate inputs from the Orinoco River, or even incongruent release of Pb isotopes from the island arc rock-derived particles must have contributed to the observed Pb isotope variability. Our results suggest that submarine hydrothermalism originating from intraoceanic island arc volcanism creates distinct geochemical environments for the dispersion of hydrothermal fluids and may be an important mechanism to supply metals of hydrothermal origin to seawater.