Clay mineral distribution characteristics of surface sediments in the South Mid-Atlantic Ridge.

Clay minerals are usually considered as important indicators to identify sources in both terrigenous and marine sediments. In particular, clay minerals in metalliferous sediments (MS) have long been studied in global oceans except in South Mid-Atlantic Ridge (SMAR) due to limited explorations. Thus, 32 MS and 34 non-MS (NMS) samples were analyzed to clarify the distribution characteristics and mineral compositions of clay minerals. All the sediments were collected along the SMAR between 12°S and 27°S. After removal of organic matter and carbonate, clay fractions (<2 µm) were investigated by X-ray diffraction (XRD) analysis. Results show that clay mineral assemblages of surface MS consist dominantly of smectite, less abundant illite, chlorite, and kaolinite in average weight percentage of 30%, 21%, 18%, and 16%, respectively. On the other hand, clay mineral assemblages in the NMS consist mainly of illite, less abundant kaolinite, chlorite, and very scarce smectite in average weight percentage of 47%, 29%, 24%, and 0.2%, respectively. The clay fractions in MS are mainly composed of amorphous or poorly crystallized Fe/Mn oxyhydroxides, clay mineral, quartz, and plagioclase. However, the counterparts in the NMS are mainly composed of well-crystallized clay minerals, quartz, and plagioclase without the presence of Fe/Mn oxyhydroxides. It is suggested that most of the illite, kaolinite, and chlorite in both MS and NMS are likely aeolian dust in origin from South Africa continent. In addition, the abundance of kaolinite dominates the clay mineral assemblage at low latitudes, where the intensive chemical weathering of continental source rocks facilitating the formation of kaolinite. In terms of smectite, it is indicated of authigenic origin in consideration of only smectite is available in several MS and the absence in NMS. Moreover, the MS samples with only smectite available are always accompanied by goethite. Therefore, it is assumed that most of smectite occurred in studied area is the results of interaction between hydrothermal Fe-oxyhydroxide, silica, and seawater. [ABSTRACT FROM AUTHOR]