太平洋海山富钴结壳铂族元素(PGE)和Os同位素地球化学及其成因意义  

Platinum group elements(PGE)and Os isotopic geochemistry of ferromanganese crusts from Pacific Ocean seamounts and their constraints on genesis

作　　者：孙晓明[1] 薛婷[1] 何高文[1] 张美[1] 石贵勇[1] 王生伟[1] 陆红锋[1] 

机构地区：[1]中山大学地球科学系

出　　处：《岩石学报》2006年第12期3014-3026,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金（批准号：40473024.40343019）；国际海底区域研究开发“十五”课题（编号：DY105-01-04-13.DY105-01-02-1）；高等学校博士学科点专项科研基金（编号：20040558049）；中国科学院广州地球化学研究所南海海洋研究所边缘海地质重点实验室研究基金课题（编号：MSGLCAS03-4）；国家教育部跨世纪优秀人才培养计划基金联合资助

摘　　要：本文分析了中西太平洋海山富钴结壳及其各主要层圈(外层、疏松层、亮煤层)和玄武岩基岩的铂族元素(PGE)和Au 含量以及 Os 同位素组成,发现富钴结壳中 PGE 和 Au 含量均较高,且变化很大,∑PGE 为(70.09～629.26)×10^(-9),平均289.48×10^(-9),Au 为(0.60～26900)×10^(-9).具三层结构的富钴结壳中,疏松层(∑PGE=(339.37～545.82)×10^(-9))和亮煤层(∑PGE=(280.09～629.26)×10^(-9))的∑PGE 明显高于外层((70.09～133.27)×10^(-9).单层结壳的∑PGE 为(83.94～479.75)×10^(-9),Au 含量普遍高于具三层结构者.结壳的∑PGE 和 Au 含量远高于太平洋多金属结核(分别为(101.57～155.83)×10^(-9)和(1～4)×10^(-9)。沉积深度和海水氧逸度的不同是导致结壳和结核中 PGE 含量明显差异的主导因素。富钴结壳∑PGE 和 Pt 与 Mn(%)之间呈明显的正相关关系,而与 Fe(%)具负相关性,与多金属结核正好相反,显示结壳中的 PGE主要赋存在水羟锰矿(δ-MnO_2)等锰矿物相中,与针铁矿(FeOOH·nH_2O)等铁矿物相关系不大,而结核中的 PGE 主要赋存在铁矿物相中。PGE 球粒陨石标准化曲线和各项参数显示富钴结壳的 PGE 和 Au 主要来自海底玄武岩的蚀变释放,部分来自铁陨石微粒等地外物质,而与海底热水活动无关。计算显示西太平洋结壳距今42.5Ma 左右开始生长,生长过程中分别在8.0Ma 和21.8Ma 处出现间断,相应形成外层、疏松层和亮煤层,其各自沉积速率为2.64mm/Ma,1.45mm/Ma 和1.06mm/Ma,相应海水的^(187)Os/^(188)Os 分别为0.948～0.953,0.599～0.673和0.425～0.536,显示外层含有较多的大陆风化尘,而疏松层和亮煤层的沉积物主要来自海底洋壳蚀变和陨石碎屑或宇宙尘等地外物质。Platinum group elements （PGE）, Au contents and Os isotopic compositions in ferromanganese crusts and their basaltic substrates from the central and west Pacific Ocean were analyzed. The results show that their PGE and Au contents are quite high and greatly varied, the ∑ PGE is （70.09 - 629.26） × 10^- 9 and the total Au is （0.60 - 26900 ） × 10^-9 respectively. In the three-layered （outer, porous and compact layers） crusts, the ∑PGE contents of porous （ （339.37 -545.82 ） × 10^-9 ） and compact （ （280.09 - 629.26）× 10^-9 ） layers are significantly higher than the outer （ （70.09 - 133.27） × 10^-9 ） layers. The single-layered crusts are also enriched in PGE（ ∑PGE =（83.94 - 479.75 ） × 10^-9 ）, and their Au contents are usually higher than the three-layered ones. In addation, ∑PGE and Au contents of ferromanganese crusts are much higher than that in the nodules （ ∑PGE = （ 101. 57-155.83） × 10^-9 Au = （ 1-4） × 10^-9 ）. Different sedimentary water depths and oxygen fugacity of the ambient sea water of the crust and nodules may be the major reason for their different PGE and Au contents. ∑PGE and Pt are positively correlated with Mn % but inversely correlated with Fe %, which is just contrary to that in nodules, suggesting that PGE in the crusts may mainly occur in manganese mineral phases such as vemadites （ δ-MnO2 ）, while the PGE in the nodules may predominantly occur in iron mineral phases such as goethites（ FeOOH·nH2O）. Chondrite-normalized patters and various geochemical parameters of PGE demonstrate that PGE and Au in the ferromanganese crusts were sourced from water/rock reaction between the basaltic substrates and seawater, and partly from extraterrestrial materials such as iron meteorite dusts, but not related to submarine hydrothermal venting. Calculation based on cobalt contents show that ferromanganese crusts in the west Pacific Ocean began to grow at about 42.5Ma, and underwent two growth hiatuses corresponding to the geo

关 键 词：铂族元素(PGE)地球化学 Os同位素 富钴结壳 多金属结核 中西太平洋海山 

分 类 号：P595[天文地球—地球化学]