索马里海盆多金属结核中关键金属富集机制及古环境记录

Enrichment mechanism of critical metals and records of paleoenvironment in a polymetallic nodule from the Somali Basin

多金属结核富含Mn、Fe、Ni、Cu等元素以及Co和REY(稀土元素和钇)等关键金属,是重要的大洋矿产资源。为了探究西北印度洋索马里海盆结核中关键金属的富集机制及古环境记录,本文使用X射线衍射仪(XRD)分析了一个结核的矿物成分,并通过激光剥蚀电感耦合等离子体质谱仪(LA-ICPMS)和电子探针(EPMA)原位分析了该结核的微区元素组成。结果表明,研究区结核内层为柱状结构,中间为掌状,外层又变为柱状。结核主要由水成型富铁层构成,但外层中还存在薄的(100~200μm)成岩-混合型富锰层。富铁层MnO平均含量为25.1%,FeO为30.4%,Co为0.26%,Ni为0.24%,Cu为0.09%,ΣREY为1981×10^(-6)。富锰层MnO平均含量为52.0%,FeO为7.40%,Co为0.09%,Ni为2.45%,Cu为0.53%,ΣREY为719×10^(-6)。富铁层主要由含铁水羟锰矿构成,而富锰层主要由Ⅰ型布赛尔矿构成。Ni和Cu主要赋存于Ⅰ型布赛尔矿,Co和REY则主要赋存于含铁水羟锰矿中,并且Co存在一个“拐点”。富锰矿物(Ⅰ型布赛尔矿)相对更富集HREE,而含铁水羟锰矿中更富集LREE。从内到外,结核中ΣREY呈下降趋势,REY的变化主要受Ce影响。结核Mn/Fe比值与δCe不相关,但生长速率与δCe显著负相关,表明生长速率可能是控制Ce异常的主要因素,仅凭Ce异常不能区分结核微区的成因及氧化还原环境。根据Co经验公式估算,索马里海盆结核形成年龄为~1.46Ma。柱状结构的开始形成可能与早更新世晚期的古海洋环境转型有关。而掌状结构开始生长于~0.6Ma,中更新世气候转型之后风尘增加使得结核中Si和Al增加,并稀释了其他成矿元素。结核富锰层可能生长于末次间冰期的亚氧化底层水环境,且具有一定对称性和渐变性,指示了氧化还原环境的逐渐改变和恢复。

Polymetallic nodules are important oceanic mineral resources,which are rich in Mn,Fe,Ni,Cu and critical metals,such as Co and REY(rare earth elements plus yttrium).In order to explore the enrichment mechanism of critical metals and records of paleoenvironment in a nodule from the Somali Basin,Northwest Indian Ocean,its mineral compositions were analyzed by X-ray diffraction(XRD)and fine scale elemental changes were analyzed in situ by laser ablation inductively coupled plasma mass spectrometry(LA-ICPMS)and electron probe microanalysis(EPMA).The results show that the structures of the nodule change outward from columnar to palmar and then to columnar.The nodule is primarily composed of hydrogenetic Fe-rich layers with averaged contents of MnO 25.1%,FeO 30.4%,Co 0.26%,Ni 0.24%,Cu 0.09%andΣREY 1981×10^(-6).However,there is a thin(100~200μm)diagenetic Mn-rich layer in the outer part of the nodule with averaged contents of MnO 52.0%,FeO 7.40%,Co 0.09%,Ni 2.45%,Cu 0.53%andΣREY 719×10^(-6).The Fe-rich layers may mainly contain Fe-vernadite,while the Mn-rich layer may mainly be composed of buseriteⅠ.Ni and Cu in the nodule are primarily enriched in buseriteⅠ,while Co and REY mainly occur in Fe-vernadite and Co shows an“inflection point”.HREE are relatively enriched in Mn-rich minerals(buseriteⅠ),while LREE are relatively enriched in Fe-vernadite.ΣREY show a downward trend from inside to outside in the nodule and the change of REY is dominant by Ce.Furthermore,there is a negative correlation between the growth rate of the nodule and itsδCe while no obvious correlation between Mn/Fe andδCe,indicating that the Ce anomaly of the nodule is likely controlled by its growth rate and cannot distinguish the fine scale origin of the nodule and the redox environment alone.According to the empirical formula of Co,the age of the nodule is~1.46Ma.The initial formation of its columnar structure maybe related to the paleooceanic transition in the late Early Pleistocene,while the palmar structure may grow since~0.6Ma when intensed dust input after MPT(Middle Pleistocene Climate Transition)caused high Si and Al contents and diluted other metallogenic elements.The Mn-rich layer may origin from the suboxic environment during the Last Interglacial and exhibits symmetry and gradient,which indicates a gradual change and recovery of the redox environment.