东南太平洋秘鲁海盆DEA区浅层埋藏型铁锰结核的矿物学和地球化学特征及成因类型

Mineralogical and Geochemical Characteristics and Genesis of Subsurface Buried Manganese Nudule from Discol Experiment Area of Peru Basin in Southeastern Pacific

本文通过对秘鲁海盆DEA区浅层埋藏型铁锰结核开展地球化学和矿物学研究,探讨了埋藏型大洋锰结核的有用组分的富集、清扫机制及成因类型。该区铁锰结核致密层和疏松层互层具有明显的多孔树状微结构,结核主要由10锰矿相、7锰矿相(水钠锰矿)和δ-MnO_(2)(水羟锰矿)等锰相矿物组成,成分上具有高Mn、低Fe和中等Ni+Cu、Mo(分别为44.79%、0.55%、0.46%、0.05%)的亚氧化成岩型特征和低的Co、Ti、Zn和REY元素(REY_(SN)均小于1)含量。铁锰结核中低含量的Mo和大部分的Ni与Mn氧化物清扫有关,其中Mo与Mn相关系数R=0.65,高Mn/Fe(>300)中Ni含量同样很高(0.34%),其他元素如Cu、Ti和REY均与Fe的羟基氧化物及铝硅酸盐相关性较高(R>0.8)。铁锰结核Fe、Ni+Cu、Mo、REY元素含量和Mn/Fe比值在分层上呈现规律性变化,致密层的Ce_(SN)负异常和疏松层的Ce_(SN)正异常进一步表明,铁锰结核的生长过程经历亚氧化-氧化成岩作用的环境条件变化,该变化致使疏松层部分组分的微弱增加和Ce富集机制的改变,而亚氧化条件下Mn氧化物的解吸附作用导致致密层中Mo含量的减少而出现与水成成因相关元素和Ce含量的协同变化。因此,研究区铁锰结核的岩石学和成分特征与表层沉积环境的特殊氧化条件具有较好的耦合关系。

The ambient enviroment of buried Marin Manganese Nodules occurrence is different from surface nodules.The studies of enrichment and scavenge mechanism of valuable elements and genetic type have been conducted deficiently on the buried Marin Manganese Nodules.In this paper,a single nodule from DISCOL Experiment area of Peru Basin in southeastern Pacific had been analyzed and studied to resolve the scientific problems above.Manganese nodule mainly consists of manganate phase including 10 manganate,7 manganate(birnessite)andδ-MnO_(2)(vernadite).High Mn,low Fe and moderate Ni+Cu and Mo contents(average 44.79%,0.55%,0.46%,0.05%)suggest that the nodule can be discriminated as sub-oxic diagenetic type.Trace elements including Co,Ti,Zn and REY(REY _(SN)>1)are extremely low as well.Mo and majority of Ni were scavenged by Mn oxide while most of other elements are bound to Fe hydroxide and aluminosilicate phase.The lamination of dense layers and porous layers in the nodule can be distinguished easily in BSE Images.The contents of Fe,Ni+Cu,Mo,REY and Mn/Fe value in two layers change regularly.The Ce_( SN )negative anomaly in dense layer and Ce positive anomaly in porous layer further indicate the sub-oxic conditions had transformed to oxic conditions during the growth of nodules in the DEA.Such transformation resulted in the slightly increase of hydrogenetic component and the change of Ce enrichment mechanism.The variation of Mo is probably attributed by the desorption of Mn oxide under sub-oxic environment.The diagenetic type manganese nodule from DEA were strongly affected by early diagenesis and the fluctuation of oxidation condition.In conclusion,the growth process of buried nodule in DEA coupled well with the specific subsurface depositional environment.