Geochemical characteristics and geological implication of ferromanganese crust from CM6 Seamount of the Caroline Ridge in the Western Pacific

Ferromanganese crusts (Fe-Mn crusts) grow very slow and can be treated as condensed stratigraphic sections recording paleo-oceanographic environmental information and local key geological events during the mineralization process. Geochronology, textures, mineralogy, and geochemistry of a Fe-Mn crust sample from the CM6 Seamount of the Caroline Ridge in the Western Pacific Ocean were analyzed by means of electron probe microanalysis and X-ray diffraction. The Fe-Mn crust shows three layers in textural characteristics from bottom to top. The lower, middle, and upper layers presence mottled, botryoidal, and columnar textures, respectively. Ferruginous vernadite, Fe hydroxide, amorphous silicate minerals, calcite, quartz, and feldspar are the main minerals of the Fe-Mn crust. Using cobalt chronometry method, the cumulative growth time of the Fe-Mn crust was determined to be 11 Ma or 25 Ma, of which 25 Ma is inconsistent with other lines of age constraint brought by dating of the substrate. The co-existence of abundant silicate minerals and bioclasts in the middle and lower layers of the Fe-Mn crust diluted the ferromanganese oxide deposits, thus affected the texture, minerals, and geochemical characteristics of the Fe-Mn crust. Variations in Mn, Co, Ni, and other elements content and the burial of opal-A recorded the expansion of oxygen minimum zone (OMZ) in the upper layer of the Fe-Mn crust. In addition, the high-reflectivity Fe-rich laminae might indicate the surrounding volcanic activity. The Fe-Mn crust sample was determined to be hydrogenic by electron probe micro-analyzer (EPMA). The findings help us understand the geochemical characteristics of the Fe-Mn crust in the Caroline Ridge Seamount in the Western Pacific and the variations of paleo-oceanographic environment clues borne by the Fe-Mn crusts.