东太平洋多金属结核富钡镁锰矿细脉蛋白石层的成因机制

The origin of the opal-layer full of todorokite veinlets in polymetallic nodules from the Eastern Pacific Ocean

为探讨东太平洋CC区多金属结核中富钡镁锰矿细脉蛋白石层的成因机制,利用矿相显微镜、电子探针和ICP-OES,对蛋白石层及其周围铁锰氧化物进行了显微结构特征和化学成分特征的研究。结果表明:高反射率矿物结晶程度较好,主要由MnO组成,具有高Mn低Fe的特征,为成岩成因的钡镁锰矿。中反射率矿物结晶程度较差,主要由MnO和FeO组成,为水成成因的水羟锰矿。低反射率矿物为一种半透明矿物,主要由SiO2组成,为生物成因的蛋白石。根据钡镁锰矿、水羟锰矿和蛋白石的成因机制,为蛋白石层及其内部钡镁锰矿细脉的形成过程建立了一个模型,共分为5个生长阶段:第1阶段,结核呈半埋藏状态在氧化环境中形成水羟锰矿;第2阶段,结核呈半埋藏状态接受SiO2胶体的沉淀作用形成蛋白石层;第3阶段,结核呈半埋藏状态在氧化环境中形成水羟锰矿的同时,其成岩作用和蛋白石的脱水作用使蛋白石层中产生大量的裂缝;第4阶段,结核呈埋藏状态在弱氧化-还原环境中发生交代作用和重结晶作用形成钡镁锰矿;第5阶段,结核呈半埋藏状态形成水羟锰矿。

There are three main manganese oxides in polymetallic nodules.They are vernadite、todorokite and birnessite.Different manganese oxides have their specific origins and growth environments.It is found that there is a special opal-layer which is full of todorokite veinlets in the polymetallic nodules from the Eastern Pacific Ocean.Microscope、EPMA （Electron Probe MicroAnalysis） and ICP-OES （Inductively Coupled Plasma Optical Emission Spectrometer） methods were used to find out the micro-structure and geochemical features of the opal-layer and its surrounding minerals,in order to investigate the origin of the opal-layer.It shows that：（1） The special opal-layer and its surrounding area are mainly composed of todorokite,vernadite and opal.（2） Todorokite has high reflectivity and Mn/Fe ratio.It is mainly composed of MnO and of diagenetic origin.（3） Vernadite has moderate reflectivity and low Mn/Fe ratio.It is mainly composed of MnO and FeO,it is of hydrogenic origin.（4） Opal has low reflectivity.It is mainly composed of SiO2 and of biogenic origin.A model was set up for the formation of the opal-layer and the todorokite veinlets inside.It could be divided into 5 stages as follows：（1）In the oxidizing environment,vernadite was formed on the surface of the half-buried nodule.（2） The dissolving of biogenic opaline silica leaded to a vast amount of colloidal silica in the upper water of the seafloor.The colloidal silica deposited on the surface of the half-buried nodule,forming the opal-layer.（3） In the oxidizing environment,vernadite was formed on the surface of the half-buried nodule.At the same time,the diagenesis of the nodule and dehydration of opal led to the formation of cracks in the opal-layer.（4） The nodule was buried by the silica-rich sediments.In the reducing environment,the todorokite veinlets formed in the previous cracks by the invasion of interstitial water from the surrounding sediments.（5） The nodule re-exposured in the water,forming vernadite on its surface in the oxidizing environment.