Geochemical and mineral composition characteristics of hydrothermal-related clay-sized surface sediments from southern Mid-Atlantic Ridge: Implications for hydrothermal depositional environment

Numerous hydrothermal activities were observed along the southern Mid-Atlantic Ridge (SMAR) in recent years. In this study, six clay-sized surface sediments collected from three hydrothermal fields (Caifan, Xunmei and Taiji) and one sample from Kaifeng seamount were analyzed for mineralogical and geochemical compositions. X-ray diffraction (XRD) results reveal that samples are mainly composed of fine-grained goethite, chlorite, and occasionally with jarosite and smectite. Interestingly, smectite not observed under XRD patterns are determined with the application of transmission electron microscope (TEM). Smectite is probably formed by interaction between Fe-oxyhydroxides (goethite) and detrital/hydrothermal silicates. Major and minor elements geochemical data indicate samples from Caifan and Xunmei are highly influenced by hydrothermal activities, and alteration of sulfides was prominent. However, it is not true for the sample from Kaifeng seamount and Taiji hydrothermal field, where are dominant by Mg-rich chlorite of volcanic rocks origin rather than aeolian. Rare earth element (REE) concentrations of both clay-sized and bulk sediment were compared to illustrate the REE characteristics and the degree of hydrothermal contribution. In the Taiji hydrothermal field and Kaifeng seamount, REE were mainly present in clay-sized fraction and chlorite may act as an efficient carrier of REE in these samples. On the other hand, in the Caifan and Xunmei hydrothermal fields, only goethite (or Fe-oxyhydroxides) plays an important role on carry of REE rather than smectite due to low abundance and poor crystallinity.