Geochemical and microtextural characteristics reflect the formation mechanics of laminated iron deposits at the Perle & Bruse and Troll Wall vent fields
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Located at the southern section of the Artic Mid-Ocean Ridge, the Jan Mayen Vent Fields (JMVF) consist of three main hydrothermal sites, the Troll Wall, Perle & Bruse and Soria Moria. These sites contain numerous Fe-deposits, located distal to high-temperature venting sites. A recent study of such Fe-deposits from the Troll Wall reports of abundant neutrophilic Fe-oxidizing bacteria (FeOB), at locations with active low-temperature venting. The same study suggests that the stratified structure, and textural and chemical variations of the laminae and layers of the deposits reflect changes in physicochemical conditions (i.e. temperature, fluid dynamics, pH, nutrient availability), which govern the formation processes, such as habitability and growth of biomineralizing FeOB, and abiotic mineralization. In this study samples of Fe-depositsfrom both the Troll Wall and Perle & Bruse vent fields are characterized by scanning electron microscopy (SEM) and geochemical analysis, with the aim of establishing possible intra- and inter-field textural and geochemical variations at these sites, and if such variations can be explained by similar physicochemical changes. The textural results reveal that the samples from both fields are built up of stacked sequences of five distinct laminae or laminated layers of different colour, thickness, porosity and microtextures, separated by internal cavities. A yellow and a light brown layer with a highly porous framework of 10-50 μm thick, 200μm to >1000μm long bundles of 0.3-1μm wide fibres, which apparently grew inward from an outer glass-like lamina and into the cavities, form the innermost layers in each sequence. The fibres were likely formed through nucleation onto extracellular polymers (EPS). Secondary mineral coating and attached twisted FeOB stalks on the bundles in the light brown layer suggest a further development from the yellow stage through different biotically and abiotically dominated stages. Clusters of 2-3 μm wide, branching tubes (Y-guys) associated with biomineralizing FeOB are mainly converging around the glass-like lamina. Massive, 5-15 μm wide, and 50-300 μm long unidirectional filamentous structures comprising most of the outermost brown and dark brown layer, are likely formed from connecting nodular, abiotic precipitates, indicating periods of poor biotic growth conditions. Lower amount of secondary mineral coating, stronger branching of Y-guys, and greater general abundance of other likely biosignatures of microaerophilic FeOB in the Perle & Bruse sample suggest better growth conditions at this site. The geochemical data confirm that the samples from both vent fields have a similar composition, with approximately 50-80 wt% Fe2O3 and 20-35 wt% SiO2, along with low contents of base metals. Mn is enriched in the surface layer relative to the interior, although the concentration is markedly lower in the Perle & Bruse sample compared to the Troll Wall sample. The enrichment likely reflects Mn-oxyhydroxides precipitation at the oxic surface during periods of hydrothermal quiescence. Thus, the variation in Mn-content between laminae at both sites suggests variations in hydrothermal input throughout the formation of the deposits at both sites, with generally a more intermittent hydrothermal activity at the Troll Wall site than the Perle & Bruse site. Chondrite-normalised REE patterns for the Troll Wall deposit are similar to that of seawater and low-temperature hydrothermal fluid previously reported for this field. However, REE patterns with similarity to Jan Mayen basalts for the Perle & Bruse deposit suggest a higher fluid temperature resulting in stronger leaching of the subsurface rocks at this site. Negative Eu anomalies indicate that the hydrothermal fluid at both sites never reached temperatures above 200-250°C. This study demonstrates that hydrothermal fluctuations and associated shifts in the position of the redox-gradient and nutrient availability likely explain the formation of the laminated structure and the activity of Fe-oxidizing bacteria in marine hydrothermal Fe-deposits. The study also demonstrates that differences in intra-and inter- field textural and chemical composition do exist between the investigated Fe-deposits, and that these differences are likely caused by variation in the venting styles and physicochemical conditions of the hydrothermal fluids. To this end, conceptual models for the formation of the Perle & Bruse and Troll Wall Fedeposits and associated microtextures are proposed.