Electron microscopy investigation of the genetic link between Fe oxides/oxyhydroxides and nontronite in submarine hydrothermal fields

The two major components most frequently found in sediments near submarine hydrothermal fields are Fe oxides/oxyhydroxides and nontronite. Physico-chemical conditions in these environments vary from site to site and even within sites as the type and intensity of the hydrothermal activity change. The investigation of Fe oxides/oxyhydroxides and nontronite is helpful to constrain the specific conditions in which they formed or evolved, and thus to recreate the history of hydrothermal vents. Microbial activity is also involved in the formation of these mineral phases, which adds the interest of revealing processes at the interface of biological and inorganic phenomena. We investigated sediments from ten seafloor hydrothermal fields, most of them Recent sediments. They are rich in Fe oxides/oxyhydroxides and/or nontronite and/or Fe-talc. SEM analysis with energy-dispersive X-ray spectroscopy revealed many morphological types, among which vermicular habits are common, with chemical composition ranging frequently between Fe oxides/oxyhydroxides and nontronite. Smooth surfaces of the investigated particles correlates with high Fe/Si ratios, and rough surfaces with low Fe/Si ratios. This feature and the fact that many particles had chemical compositions (Si, Fe, Mn, Mg, Al) in a continuous line linking Fe oxide and nontronite are compatible with the in situ reaction of Fe oxides/oxyhydroxides as a frequent mechanism of nontronite formation. This process involves the reaction of Fe oxides/oxyhydroxides with either silica polymers adsorbed on their surface or Si-Fe amorphous phases precipitated on the same surface. However, it was not possible to establish a pseudomorphic replacement (of Fe oxides by nontronite) as no nontronite grains were found reproducing habits observed in grains with large proportion of Fe oxides/oxyhydroxides. All particle morphologies can have a range of surface roughness and composition. Chemical composition consistent with nontronite was represented mainly by small particles (< 1 mu m) and flakes or veil-like habits. The frequent vermicular habit caused by precipitation on Fe-oxidizing bacteria was in line with previous findings indicating this as an important process of precipitating a wide range of Fe-Si phases. Overall, our data suggest that in situ replacement of Fe oxides/oxyhydroxides by nontronite takes place in all types of particles but that the replacement does not preserve the original particle morphology.