Young volcanism and related hydrothermal activity at 5°S on the slow-spreading southern Mid-Atlantic Ridge

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge

Reflectance spectra of terrestrial barites conducted in the Planetary Emissivity Laboratory (PEL)

The most important source for barite on Earth is often associated to sedimentary exhalative sulfide deposits (SedEx) and volcanic environments generated by hydrothermal activity. BaSO4 is also related to epithermal deposits in volcanic rocks and is a common gangue mineral in veins and fractures due to wall rock alteration [1]. Thus, these minerals often reflect close environmental compositions and are therefore interesting for the understanding of volcanogenic and hydrothermal activity on terrestrial planets. In this study, we present our first results of selected samples from an ongoing spectral and geochemical investigation on barium sulfates (barite minerals, provided by Deutsche Baryt-Industry, Bad Lauterberg). We analyzed our samples with bidirectional reflectance measurements conducted in the Planetary Emissivity Laboratory (PEL) at the German Aerospace Center (DLR) in Berlin, Germany

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

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 ($\varepsilon$Nd = +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 ($\varepsilon$Nd = ~10.7). These crusts also show high 176Hf/177Hf (up to 0.283102), low 87Sr/86Sr (up to 0.7069), and low 1870s/1880s (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