Fluid-related manganese deposits at the incoming plate of the Southern Mariana subduction zone

Abstract

Manganese is one of the most abundant transition metals in marine systems, controlling the behaviors of a variety of important elements in oceans worldwide. Deposited manganese minerals in marine systems mainly belong to poorly crystalline manganates or sometime tecto-manganates in the form of crusts and nodules origin from ambient bottom seawater, sediment porewater, or ascending hydrothermal fluids. Here, we report the discovery of two kinds of fluid-related manganese deposits on the incoming plate of the Southern Mariana subduction zone. Hausmannite that rich in copper and zinc with a strong negative Ce anomaly is identified in one sample. Notable features of this unusual manganese mineral indicate that reduced fluid derived from the nearby fluid discharge site reacted with original manganese oxides and then precipitated as hausmannite in the sub-oxic subsurface. However, another fluid derived sample located approximately 60 m away, shows normal hydrothermal mineral and elemental features. The atypical and diverse fluid-related manganese deposits more likely indicate multiple episodes of fluid activities at the subduction front. As bend-faults are proven to induce the fluid discharge at the outer-rise region of the incoming plate, manganese deposits in the present study suggest that an appreciable amount of manganese in the upper lithosphere of the incoming plate may be discharged into seawater through fluid activities before subduction. As such, the mass of manganese subducted into the subduction zone may be much lower than previously expected.