Mixed fluid processes in Fe Mn dendrite formation and associated carbon and nickel isotope fractionation

Dendritic iron- and/or manganese-rich microstructures, often referred to as “microstromatolites,” are commonly observed in carbonate veins in the deep subsurface. However, the mechanisms responsible for their formation, particularly the role of microbial processes, remain incompletely understood. One hypothesis suggests that Fe- and Mn-enriched fluids, sourced from submarine hydrothermal vents and circulating through mafic or ultramafic rocks, lead to the precipitation of manganese dendrites within open fractures. Microbial activity has been proposed as a contributing factor, particularly due to its ability to catalyze Mn 2+ oxidation to Mn 4+ at rates significantly faster than abiotic processes under ambient conditions. Such microbial mediation often results in the formation of poorly crystalline Mn oxide phases, which are commonly associated with biologically mediated oxidation. These disordered Mn oxides, frequently observed in natural settings, suggest a microbial contribution to mineral precipitation, particularly in environments where redox gradients and fluid-rock interactions are prominent. Because manganese oxides are an important sink for Ni in marine systems, stable Ni isotope analyses may offer valuable insights into their formation. Biological activity in laboratory systems is known to fractionate Ni isotopes, producing negative δ 60 Ni values, while abiotic interactions with Mn oxides can result in a range of isotopic signatures. In this study, we show that manganese-rich dendrites likely formed through the interplay between organic matter, oxidizing fluids and viscous serpentine muds, resulting in the fractionation of both carbon and nickel isotopes. The moderately negative δ 13 C and δ 60 Ni values, together with the presence of organic matter, suggest a mixed formation pathway involving both abiotic mineral precipitation and biologically mediated processes. One plausible mechanism involves the nucleation of Mn oxides on nanoparticulate “seeds,” which could include both abiotic particles, organic matter, microbial cells and their metabolic byproducts. Understanding the formation of Fe Mn dendrites is key to interpreting the biogeochemical cycling of essential elements like iron, manganese, and nickel. Due to its redox flexibility, Mn forms highly reactive oxides that effectively scavenge trace metals such as Ni, Co, Fe, and Cu, facilitating their removal from seawater and incorporation into marine minerals. Our findings underscore the complexity of Fe Mn oxide formation and point to the combined influence of abiotic fluid dynamics and microbial processes. This improves our ability to interpret geochemical signatures in both modern and ancient environments and enhances the utility of stable isotope systems in reconstructing past ocean conditions and elemental cycling.