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

Abstract

[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 10 Be 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 Nd of up to 0.512817 (eNd = +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 (eNd = À10.7). These crusts also show high 176 Os (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.

Iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids

“…The estimated AMn flux is much higher in the Montserrat sediments (1.5 lmol cm À2 yr À1 ) compared to Crozet (0.01 lmol cm À2 yr À1 ), and is higher than the average dissolved input observed on continental margins ($0.4 lmol cm À2 yr À1 ) (Berelson et al, 2003). The AMn flux from sediments around Montserrat indicates that it is likely an important local source of dissolved Mn, consistent with the presence of abundant low temperature ferromanganese crusts in the area (Frank et al, 2006). Although Montserrat-type sediments may represent an important source of dissolved Mn in other island arc settings, their contribution to global seawater Mn budgets is not likely to be significant in comparison with other fluxes (e.g.…”

Submarine volcanoes of the Kolumbo volcanic zone NE of Santorini Caldera, Greece

“…Frank et al, 2006). The only other cone in the NE field that showed any indication of this type of venting was cone 7 and the occurrence there appeared to be very much less abundant.…”

“…Adjacent to some of the areas with black precipitates there were groupings of dark mollusk shells suggesting low level venting of hydrothermal fluids (e.g. Frank et al, 2006). Upslope, the sediment becomes coarser and random decimeter-scale blocks can be observed on the peak.…”

“…The surfaces of many of the outcrops are moderately to extensively colonized by biological organisms such as corals, sponges and other invertebrates. A preliminary conclusion about this cone based on observed sediment cover and lack of fresh volcanic outcrops is that it has not been recently active but the occurrence of possible hydrothermal venting suggests potential for future activity (Frank et al, 2006).…”

Deep-sea coral aragonite as a recorder for the neodymium isotopic composition of seawater

“…Since there is the possibility of a change in strength of the intermediate water-mass flow off Florida during the Holocene (for example, see Lynch-Stieglitz et al (2009)), we exclude this coral from our calibration. In principle, more radiogenic Nd isotopic compositions in intermediate depth seawater off Florida could originate from Antarctic Intermediate Water (e.g., Jeandel, 1993;Pahnke et al, 2008) or the Gulf of Mexico through the Florida Straits (Caribbean seawater probably has an epsilon Nd close to À10.7; Frank et al, 2006).…”