Pourbaix diagrams to decipher precipitation conditions of Si-Fe-Mn-oxyhydroxides at the PACMANUS hydrothermal field

Abstract

Utilizing Si, Fe and Mn concentrations within the end-member PACMANUS hydrothermal fluid, Si-Fe-Mn-H2O Pourbaix diagrams were constructed at 300°C and 25°C. The Pourbaix diagrams show that the main Si, Fe and Mn oxides species precipitating from the hydrothermal fluid were SiO2, Fe(OH)3, Fe3(OH)8, Mn3O4, and Mn2O3 at 25°C. During mixing of hydrothermal fluid with seawater, SiO2 precipitated earlier than Fe-Mn-oxyhydroxides because of the lower stability boundary. Then Fe(OH)2 precipitated first, followed by Fe3(OH)8 and Fe(OH)3, and last, small amounts of Mn3O4 and Mn2O3 precipitated. Fe(OH)3 was readily deposited in alkaline solution with little influence by Eh. There were many Si-Fe-Mn-concentric particles in the polished sections of the massive precipitates collected from PACMANUS. In the concentric nucleus and ellipsoid, Si oxides precipitated first before the hydrothermal fluid had mixed with seawater. In the concentric nucleus, after the precipitation of Si oxides, the increase of pH and Eh promoted the precipitation of Mn oxides around the Si oxides. In the large ellipsoid, the precipitation of Fe was divided into two periods. In the early period, increase of pH value of hydrothermal fluid produced by low-temperature convection and an input of a small volume of seawater promoted a small amount of Fe(OH)3 to precipitate in the Si-rich core. In the late period, after complete mixing with seawater and the resultant fluid was close to neutral or slightly alkaline in pH, Fe(OH)3 was easily precipitated from the solution and distributed around the Si-rich core.