The Zonal Migration Of The Seamounts In Middle-West Pacific During Cenozoic And Its Influence On The Formation Of Cobalt Bearing Crust

The thesis is to discuss the zonal migration of the seamounts in Middle-West Pacific during Cenozoic and its influence on the formation of cobalt-bearing crusts with two cobalt-bearing crusts CLD34-2 and CLD50 from seamount Lamount in the Marcus-Wake Seamount clster as its study samples by analyzing their main elements, trace elements and rare elements and combining the Os isotope stratigraphy method and the empirical formula of Co contents to confirm the ages of the samples. The following are the major findings and results obtained:(1) Cobalt-bearing crusts in Middle-West Pacific have a typical three-tier architecture:the dense underlayer, the loose middle layer and the dense upper layer. The ages of each layer in different crusts are approximately the same and the growth period of the underlayer is in stage I (55～48Ma), the middle layer in stage II (48-12Ma), while the upper layer in stage 111(12Ma～0Ma).(2) The stages are closely related to the Cenozoic ocean evolvement:the climate in stage I was warm, so the crusts formed a phosphated parallel structure. During stage II, with the polar temperature dropping, the latitudinal temperature gradient increasing, the biological and detrital components of crusts were high and the structure became taxitic. The most visible feature of this stage is the existence of a common growth hiatus. During stage Ⅲ, the polar temperature became colder and the ocean circulation increased, so the productivity of the ocean was much higher. Crusts have moved away from the equator productive area, gradually moving to the north Pacific Ocean diatoms high productivity area. Therefore, Si content was on the rise and the biological components(Cu,Ba,Zn) and detrital components(Al,Ti,K) became lower.(3)In terms of the hiatus between late Eocene to middle Miocene in stage Ⅱ, its latitude range was quite the same as the range of intertropical convergence zone and moving southward since 34Ma. With the geochemistry records of crusts, it is possible to deduce that the reason leading to the hiatus is the low productivity and the high detrital imput.