Mineralogical characterization and formation of Fe-Si oxyhydroxide deposits from modern seafloor hydrothermal vents

We have studied mineralogical characteristics of Fe-Si oxide precipitates from hydrothermal fields of the Valu Fa Ridge, Lau Basin, especially the role that the neutrophilic Fe-oxidizing bacteria played in their formation, using various analytical techniques (XRD, SEM, EPMA, TG/DTA, and FTIR). According to this examination, the Fe-Si oxide formation can be divided into two stages. At the initial stage, the Fe-oxidizing bacteria bound and oxidized Fe2+ into Fe3+ to fix CO2, triggering precipitation of Fe-oxyhydroxide (ferrihydrite) and construction of a loose network of Fe-rich filaments. Subsequently, the decreased porosity of the network resulting from the gradual growth of the filaments led to a decline in the mixing between seawater and the hydrothermal fluids. Then the conductive cooling of the network resulted in saturation of the dissolved Si with respect to amorphous silica. As a result, significant precipitation of opal-A occurred through inorganic polymerization. However, part of the silica was immobilized by bonding to Fe-OH functional groups and yielded unpolymerized silica, which is characterized by Fe-O-Si bond. Owing to the incorporation of Si into the ferrihydrite structure and its adsorption on the ferrihydrite surface, the modern hydrothermal Fe-Si oxides are thermally stable. DSC measurements indicate the full segregation of cristobalite from hematite at about 800 °C in an O2 atmosphere. These observations indicate that primary alternating Si- and Fe-rich layers may be absent in the Archean ocean and that alternating bands in BIFs represent a diagenetic process; our work thus provides a potential clue that can be used to unravel the precipitation and diagenetic mechanisms of Precambrian banded iron formations (BIF).