Formation of Fe-Mn-Si oxide and nontronite deposits in hydrothermal fields on the Valu Fa Ridge, Lau Basin

Hydrothermal Fe-Mn-Si oxides and nontronite are pervasive in the Hine Hina, Vai Lili and Mariner hydrothermal fields along the central Valu Fa Ridge, Lau Basin. Morphometric and mineralogical analyses reveal that the iron-rich filaments are the most important constituents of these Fe-Mn-Si oxide deposits. Both the morphologies and chemical composition of the filaments indicate that neutrophilic Fe-oxidizing bacteria have played a key role in the formation of these deposits. A key process of the formation of these deposits is the creation of a complicated filamentous network in which a series of metabolic activities and passive sorption and nucleation processes occur. The precipitation of dissolved Si in unsaturated and saturated states leads to a “two-generation” growth model in the hydrothermal vents. The precipitation of amorphous opal occurs in a relatively narrow temperature range (41.1-42.9 °C) based on oxygen isotope analyses, indicating a fast precipitation rate of opal-A when conductive cooling of the hydrothermal fluid occurrs. Patchy nontronite in the Mariner fields is a product of the direct precipitation from hydrothermal fluids at a temperature of ∼87.9 °C, whereas the scattered nontronite at the Hine Hina field is the product of the replacement of hydrothermal Fe-Si oxides at a temperature of ∼46.2 °C.