Geochemical and Sr‐Nd‐Pb‐Fe Isotopic Constraints on the Formation of Fe‐Si Oxyhydroxide Deposits at the Ultraslow‐Spreading Southwest Indian Ridge.

Modern Fe‐Si oxyhydroxide deposits occur in global marine hydrothermal vent sites. Despite their role as biogenic substrates and potential ore resources, much remains unknown about their formation processes. Here, we apply analyses of major and trace elements as well as Sr‐Nd‐Pb‐Fe isotopes combined with 238U‐230Th dating to Fe‐Si oxyhydroxides obtained from several hydrothermal fields along the Southwest Indian Ridge. These mineralized oxyhydroxides primarily consist of poorly crystalline two‐line ferrihydrite and amorphous opal‐A, with lesser amounts of nontronite and birnessite. The ubiquitous and characteristic Fe‐rich ultrastructures in the oxyhydroxides directly indicate microbial activity. The 238U‐230Th dating constrains their crystallization ages from ca. 11,873 to 384 years old. The seawater‐like 87Sr/86Sr and varying 143Nd/144Nd ratios underline a high proportion of seawater mixed with hydrothermal fluids. The radiogenic Pb isotopic patterns suggest a primary derivation of Pb leached from substrate basalts and to a lesser extent Pb from seawater. Stable iron isotopic compositions for different oxyhydroxides display a remarkable range between −1.47 and 0.82‰, which were interpreted as reflecting the fractionation processes during the formation of the deposits under evolving depositional redox conditions. The partial oxidation of Fe(II) and the subsurface removal of isotopically heavy Fe oxyhydroxides are suggested to play a vital role in shifting the Fe isotopic signature toward more negative values. Given that these Fe‐Si oxyhydroxide deposits exhibit features similar to certain ancient iron formations (IFs), Fe isotope systematics of these deposits may hold significant potential for fingerprinting the biological Fe oxidation processes that drove IF deposition on early Earth. Plain Language Summary: This study introduces a comprehensive examination of Fe‐Si oxyhydroxide deposits in marine hydrothermal vent sites along the Southwest Indian Ridge. Applying innovative techniques of major and trace element and Sr‐Nd‐Pb‐Fe isotope analysis combined with 238U‐230Th dating, the investigation uncovers new insights into the composition, age, and origin of microbial Fe‐Si oxyhydroxide ultrastructures. Notably, these deposits are found to be primarily composed of ferrihydrite and amorphous opal‐A, with ages ranging from ca. 11,873 to 384 years old. Our study provides evidence for their low‐temperature hydrothermal origin, with geochemical characteristics highlighting rare earth element patterns and radiogenic Sr‐Nd‐Pb isotopic compositions. Of particular significance is the discovery of a wide range of stable iron isotopic compositions, shedding new light on the depositional redox conditions. Our data, in concert with previous studies, reveal similarities between biogenic Fe‐Si oxyhydroxide deposits and ancient iron formations (IFs). This link indicates the potential use of Fe isotope systematics in tracing Fe oxidation processes and the biosignatures that may have shaped IF deposition on early Earth. These novel findings contribute fresh perspectives to the understanding of low‐temperature hydrothermal deposit geochemistry and the search for geological biosignatures. Key Points: We present a comprehensive Sr‐Nd‐Pb‐Fe isotope investigation of modern Fe‐Si oxyhydroxide deposits in marine hydrothermal vent sitesSr‐Nd‐Pb isotope variations in Fe‐Si oxyhydroxides were produced by varying degrees of seawater mixing with diffuse hydrothermal fluidsFe isotope systematics of Fe‐Si oxyhydroxides can provide key information for Fe oxidation processes during the formation of the deposits [ABSTRACT FROM AUTHOR]