Formation of ferromanganese crusts on northwest intertropical Pacific seamounts: electron photomicrography and microprobe chemistry

摘要: Abstract Seven ferromanganese crusts from the northwest intertropical Pacific seamounts were analyzed for photomicroscopic growth structures, microprobe chemistry, and ages based on Co-chronometer rate. The Marshall Islands are thick divided into phosphatized lower older nonphosphatized upper younger generations: crust consists of compact laminations columns impregnated with carbonate fluoapatite (CFA), whereas is characterized by porous botryoids δ-MnO2 Fe oxyhydroxide. Federated States Micronesia (FSM) Palau thin often incorporated inorganic opal-A in uppermost part, comprising generation. Some show scours fractures. Although has been interrupted mass failure slope sediments, estimated to have grown at rate about 3 mm/Ma since middle Eocene late Oligocene during host located beneath equatorial zone high productivity. Prolonged infiltration oxygen minimum (OMZ) water shallower redistributed composition precipitating CFA, enriching subsequent amounts Mn Ni, removing some Co. formed slower (about 2 mm/Ma) under stronger influence bottom-water circulation north zone, concentrating abundant In part crusts, siliceous skeletons transform burial Si-rich oxyhydroxide, suggesting that biosilica diagenesis can enhance growth.