Geochemical studies of Fe-Mn-oxides from the Southwest Indian Ridge: A new tool for the exploration of seafloor hydrothermal activity on off-axis

The hydrothermal circulation process along the spreading mid-oceanic regime is responsible for the major exchange of mass and energy between the lithosphere and ocean waters through chemical species. High-temperature and chemically-rich fluids issuing out of the hydrothermal vents modify the surrounding seawater (by adding and removing elements) and form thick clouds of particles that settle on the sediments and rocks around. The consequent processes lead to the formation of high, base (copper, zinc, lead) and precious (gold, silver), metal-rich hydrothermal sulfide deposits. Usually, an integrated multi-disciplinary approach involving geophysical, geological, and geochemical studies of the seafloor and the water column is engaged effectively in exploring hydrothermal plumes and associated deposits. However, it is considered a very difficult task owing to the small vent size, which is of few meters in water depths of more than one kilometer. Water column studies are tracer-based (turbidity, dissolved gases such as
³
He and methane, dissolved metals such as manganese, iron) and are effective in identifying the hydrothermal plumes (classified as high-temperature and low-temperature) that are issuing out of hydrothermal vents.Fe-Mn-oxides (hereafter called oxides), which are formed due to the precipitation of either chemically enriched fluids or seawater onto the surrounding rocks, is considered as one of the best archives of water column processes. Hence the geochemical studies of these oxides may allow us to understand the seafloor hydrothermal processes, and thus can be used as a tracer. Bulk geochemical composition of oxides, collected at several locations along the ultraslow-spreading Southwest Indian ridges (SWIR) where plate tectonics dominate over the volcanic process, are studied to identify the seafloor hydrothermal activity. The oxides are dominated by vernadite with traces of quartz and calcite minerals and high concentrations of major (Fe, Mn), trace (Co, Ni, Cu & Zn), and rare earth elements indicate that the particles incorporate these elements from seawater (present as ionic species) mainly through scavenging processes following complex mechanisms including electrostatic attraction and chemical interaction and classified as hydrogenous. On the other hand, some of these oxides are characterized with negative or no cerium anomalies in the shale normalized REE pattern indicates the influence of hydrothermal plumes during their formation, and are classified as mixed-type (hydrogenous and hydrothermal). High REE fractionation and excess cerium (Cexs) calculations suggest that the mixed-type oxides have a significant contribution from high-temperature hydrothermal plumes. It is exciting to observe that some of these mixed-type oxides were collected far away from the ridge axis.The phase distribution of REE-Y shows that 70–83% of total REE-Y is associated with the Fe-oxyhydroxide phase in SWIR oxides and indicates the large-scale dispersion of hydrothermal iron. On the contrary, the hydrogenous oxides [1] found elsewhere (for example, in the Andaman basin and Central Indian Ocean basin) show near equal distribution of REE-Y (50% of the total REE-Y is associated with the Fe-oxyhydroxide phase and 50 % with the Mn-oxide phase). Hence, the distribution of iron and manganese in major geochemical phases; exchangeable/carbonate, Mn-oxide, Fe-oxyhydroxide, and residual silicate are helpful in understanding the solid phase portioning behavior. Irrespective of their nature (hydrogenous or mixed-type), the phase distribution of manganese shows that more than 90% of total manganese is associated only with the Mn-oxide phase. On the contrary, iron is associated with three phases (Mn-oxide, Fe-oxyhydroxide, and silicate) except the carbonate phase. About 58-84% of the total iron is associated with the Fe-oxyhydroxide phase followed by 3-31% with silicate and 8-15% with the Mn-oxide phase. An average of 70% of the total iron is associated with the Fe-oxyhydroxide phase, which is commonly observed in hydrogenous oxides, with an exception of 10% associated with the Mn-oxide phase because of iron intergrowth with Mn-oxide. However, it is surprising that an average of 16% iron is associated with the silicate phase. Such phase association of iron with silicates is commonly observed in Fe-Mn-Si-oxyhydroxides precipitated by diffuse venting of hydrothermal fluids where iron and silica are co-precipitated at low-temperature (<80 °C). The possibility of iron-sulfides or crystalline iron-oxides (magnetite) in these oxides cannot be ruled out because they also fall in the silicate phase of the leaching experiment, which follows simple four-phase sequential leaching of carbonate, Mn-oxide, Fe-oxyhydroxides, and silicate minerals.Therefore, the geochemical studies of Fe-Mn-oxides from the SWIR provide primary evidence for high-temperature hydrothermal activity along the ridge with particular inference on off-axis hydrothermal activity. Detailed iron-speciation experiments may be required to understand its association in different mineral phases. A special focus on the off-axis regions of the ultraslow-spreading SWIR is much needed to understand the biogeochemical processes associated with hydrothermal circulation.