Mineralogy and geochemisry of ferromanganese crusts from Guanshi Seamount in the eastern South China Sea

Although the mineral composition and geochemistry of ferromanganese crusts in the open ocean are well understood to researchers, those on seamounts in a marginal sea remains little known. In this study, two ferromanganese crust samples are collected from the Guanshi seamount located in the eastern South China Sea (SCS). We analyzed the mineral composition, microstructures, and major elements contents of the ferromanganese crusts, by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and electron microprobe, respectively. Both of the two ferromanganese crusts are dominated by vernadite with a small amount of geothite, while the detrital minerals dominated by quartz and feldspar. The two crusts are all striped in microstructure, with pores of 10-15 μm and holes of 0.5 μm in diameters. Compared with those from the open ocean, the crusts from Guanshi seamount is higher in Mn, due to the volcanic eruption near the Manila Trench, and lower in Cu, Co and Ni contents due to the higher deposition rate and higher adsorption of organic complex in the South China Sea. Mineral composition and element ratios indicate that the two crusts are both hydrogenetic, without obvious phosphatization. The element ratios and correlation of elements show that Mn mainly deposited from the dissolved Mn supplied by volcanic eruption and the weathering of basalt formed in the post-spreading stage of the South China Sea. Fe and Si elements are both derived from volcanic, biogenic and terrestrial materials, and also affected by the adsorption of tubular worms. Two crusts from the Guanshi seamount in the South China Sea are formed by the slow accumulation of dissolved chemical elements of seawater affected by upwelling under an oxygen enriched environment. The temporal variations of element contents in the ferromanganese crusts reflect paleoenvironmental evolution history of the South China Sea.