| Ferromanganese crusts and nodules are marine mineral resources rich in elements such as Mn,Fe,Co,Ni,and rare earth elements,recording key paleoceanographic information.In this paper,the ferromanganese oxides(ferromanganese crust and ferromanganese nodule)developed in different environments on Caiwei Guyot of the Western Pacific region were studied,and the ferromanganese crusts of the Western Mariana Ridge were systematically studied for the first time.The microstructure,mineralogical characteristics,and geochemical characteristics of ferromanganese crusts and nodules in the two regions were analyzed using scanning electron microscopy,X-ray diffraction(XRD),inductively coupled plasma emission spectrometry(ICP-OES),X-ray fluorescence spectroscopy(XRF),inductively coupled plasma mass spectrometry(ICP-MS),and electron probe microanalysis(EPMA).The genetic types and formation mechanisms of ferromanganese crusts and ferromanganese nodule in different geological tectonic environments were discussed.Finally,under the determined chronological framework,the geochemical compositions of ferromanganese crusts and ferromanganese nodule were used to reconstruct Pacific Deep Water.The main research results are as follows:(1)The ferromanganese crusts developed on the mountaintop edge of the Caiwei Pingding seamount are plate shaped crusts,while the ferromanganese nodules developed on the slope of the seamount are plate shaped crusts.The thickness of plate-like crust is relatively large,with dense and loose layers distributed alternately inside.The core of ferromanganese nodules is bedrock,and the outer core is dense and loose layers.According to the surface structure,the strong hydrodynamic conditions in the growth environment of ferromanganese crusts are higher than that of the ferromanganese nodule.The main microstructures of ferromanganese crusts and ferromanganese nodule are columnar,layered,dendritic,and mottled structures,and the columnar structure,dendritic structure,and mottled structure are formed under strong hydrodynamic conditions.The ferromanganese crusts on the Western Mariana Ridge are gravel shaped crusts with a relatively thin thickness.The main microstructures are columnar structure,dendritic structure and mottled structure with significant differences in microstructure characteristics at various stages.(2)The main mineral compositions of ferromanganese crusts and nodule on Caiwei Guyot are Fe-vernadite,anorthite,albite,and quartz.Todorokite,goethite and phillipsite are developed in some individual layers.Fe-vernadite is the main mineral of hydrogenetic ferromanganese crusts and the ferromanganese nodule,while todorokite is considered to be the main product of diagenesis.Quartz,anorthite and albite are mostly derived from aeolian dust;Goethite is closely related to marine productivity.The mineral compositions of ferromanganese crusts on the Western Mariana Ridge are similar to those of the ferromanganese crusts on Caiwei Guyot,which are mainly Fe-vernadite,quartz,anorthite and albite.However,no todorokite is developed during its growth,indicating that the growth process is not controlled by diagenesis.In addition,calcite,sanidine and illite are developed in ferromanganese crusts.The high contents of illite,quartz,albite and anorthite indicate that compared with ferromanganese crusts in Caiwei Guyot,aeolian dust may have a greater impact on the growth process of ferromanganese crusts on the Western Mariana Ridge.The development of ilmenite indicates that the formation of ferromanganese crusts on the Western Mariana Ridge may be controlled by volcanic activity.(3)The ferromanganese crusts and the ferromanganese nodule on Caiwei Guyot are rich in metal elements such as Mn,Fe,Co,and Ni.The element contents in each layer are not constant,but change with the surrounding environment during the growth process.The high content of Cu in the ferromanganese nodule may be related to water depth and diagenesis.Both ferromanganese crusts and the ferromanganese nodule have high levels of REY,and each layer of ferromanganese crusts and the ferromanganese nodule has obvious positive Ce anomalies and negative Y anomalies,with almost no Eu anomalies,suggesting both are of hydrogenetic origin and their ore-forming elements come from oxygen-rich seawater.The geochemical characteristics of ferromanganese crusts on the Western Mariana Ridge are similar to those on Caiwei Guyot,and belong to the type of hydrogenetic ferromanganese crusts.The internal micro layer has a high Mn/Fe ratio,which may be related to diagenesis under the internal suboxide environment.(4)The chronological framework of ferromanganese crusts and the ferromanganese nodule on Caiwei Guyot has been established since the Late Cretaceous.It is found that the trend of geochemical compositions changes in ferromanganese crusts and the ferromanganese nodule and is closely related to the Cenozoic marine evolution process.When the global climate becomes colder,the intensity of Pacific Deep Water increases,the range of the oxygen minimum zone in the guyot narrows,and the seawater in the environment where ferromanganese crusts grow becomes more oxidizing,making it more suitable for the mineralization of ferromanganese crusts.The formation process of the ferromanganese nodule on Caiwei Guyot is divided into two stages.During the first stage,from the Middle Miocene to the Late Pliocene,the global cooling caused its growth process to be affected by the Pacific Deep Water rich in dissolved oxygen,and the oxidizing nature of the seawater environment around the ferromanganese nodules continued to increase.The contents of Mn,Fe and Co in ferromanganese nodules in the second stage have decreased to varying degrees compared with those in the first stage,and diagenesis occurred in the ferromanganese nodule under the condition of sub oxidation.The Mn/Fe ratio of the young layers of the ferromanganese nodule is greater than 2,indicating that micro or nano scale diagenetic processes may have developed within the nodules,which may be related to biological activities on the slope leading to the formation of a micro reducing environment inside the ferromanganese nodule.(5)On the basis of determining the chronological framework,the growth process of ferromanganese crusts on the Western Mariana Ridge was explored.Its growth process was restricted by the closure of the Panama Seaway,Northern Hemisphere Glaciation,Mid-Pleistocene Transition and other paleoceanographic events.According to the mineralogy and geochemical characteristics of ferromanganese crusts,the deep water environment of the Western Mariana Ridge since 2.84 Ma can be divided into three stages.In the first stage(2.84-2.22 Ma),when Northern Hemisphere Glaciation occurred,the global climate became colder.At this time,the content of dissolved oxygen in Pacific Deep Water was higher,the range of oxygen minimum zone was reduced,ocean productivity was reduced,and aeolian dust gradually entered the Pacific Ocean.After the Northern Hemisphere Glaciation occurred,the oxidation of seawater decreased,the range of oxygen minimum zone gradually increased,and the input of aeolian dust gradually decreased;In the second stage(2.22-1.21 Ma),the intensity of Pacific Deep Water increased,the range of the oxygen minimum zone gradually decreased,the dissolved oxygen content in seawater was high,and aeolian dust continued to enter into the Pacific.In the third stage(1.21 Ma～0),during the Mid-Pleistocene Transition,the oxygen minimum zone contracted,and seawater oxidation increased.Diagenesis under the condition of suboxidation occurred in some micro layers inside the ferromanganese crusts.After the climate fluctuations in the Mid-Pleistocene Transition,the drought degree of Central Asian ardification increased,the input of aeolian dust increased,the range of the oxygen minimum zone in the ocean widened,and the seawater oxidation decreased,indicating a decrease in the intensity of Pacific Deep Water. |
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