前寒武纪条带状硅铁建造的形成机制与地球早期的大气和海洋  

Formation Mechanism of Precambrian Banded Iron Formation and Atmosphere and Ocean during Early Stage of the Earth

作　　者：李延河[1] 侯可军[1] 万德芳[1] 张增杰[1] 乐国良[1] 

机构地区：[1]国土资源部成矿作用与资源评价重点实验室,国土资源部同位素地质重点实验室,中国地质科学院矿产资源研究所,北京100037

出　　处：《地质学报》2010年第9期1359-1373,共15页Acta Geologica Sinica

基　　金：国家自然科学基金(编号40273008;40803001;40203003);地质调查项目(编号1212010916067);公益行业科研专项(编号200811114);地质过程与矿产资源国家重点实验室资助的成果

摘　　要：前寒武纪条带状硅铁建造(BIFs)是地球早期特有的化学沉积建造类型,记录了当时大气和海洋的化学成分、氧化还原状态及演化。本文系统测定了华北地台条带状硅铁建造的硫硅氧同位素组成。不同时代和不同类型条带状硅铁建造中石英的硅同位素组成非常相似,强烈亏损30Si,δ30SiNBS-28大部分位于-2.0‰～-0.3‰之间,平均-0.8‰;硅铁建造中石英的δ18OV-SMOW相对较高,8.1‰～21.5‰,平均13.9‰;二者均与现代海底黑烟囱、泉华及热水沉积硅质岩的硅氧同位素组成相似。在同一样品中,磁铁矿条带中石英的δ30SiNBS-28普遍低于相邻硅质条带中石英的值,而δ18OV-SMOW刚好相反,反映了硅铁建造沉积时的初始特征。BIFs中硫化物的δ34SV-CDT变化范围很大,-22.0‰～+11.8‰,但大部分集中分布在0值附近。Δ33S=-0.89‰～+1.2‰,显示出了明显的硫同位素非质量分馏特征,说明当时大气氧浓度很低。与火山活动关系密切的Algoma型硅铁建造的Δ33S多为负值,而远离火山活动中心的Superior型硅铁建造的Δ33S多为正值。提出无论是Algoma型,还是Superior型BIFs都是由海底热液喷气作用形成的。富含溶解硅和铁的热水溶液喷发到在海底以后,由于温度突然下降,硅酸H4SiO4在海水中达到过饱和状态,导致SiO2首先沉淀,形成硅质层;随着热水溶液与海水的不断混合,温度不断降低,Eh值不断升高,Fe2+逐渐被氧化生成Fe3+随后沉淀,形成富铁层。一套硅铁韵律层代表了一次大的海底喷气活动;海底热液喷气的周期性活动形成了规律性的硅铁韵律层。BIFs的广泛分布和硫同位素非质量分馏效应的普遍存在,表明当时大气氧水平很低,可能不足现在氧水平的1‰;火山和海底喷气活动非常强烈,海水温度较高,呈酸性,pH值在3.0～5.5之间;海洋中可溶解硅H4SiO4和Fe2+的浓度很高;而可溶硫酸盐的浓度极低,<1mM。早元古代(1.8Ga)以后海洋硫Isotopic compositions of S,Si and O of Precambrian BIFs at the North China Craton （NCC） were measured systematically. The δ^30SiNBS-28 values of quartz in BIFs of different types and ages are low with most ranging from-2.0‰ to-0.3‰,similar to the δ^30SiNBS-28 values of modern submarine black smoker and sinter. The δ^30SiNBS-28 values of quartz contained in banded magnetite are lower than those of neighboring siliceous band for the same sample,while this goes the other way round for δ^18OV-SMOW. The δ^34SV-CDT values in the sulfides of BIFs vary greatly from-22.0‰-＋11.8‰,but mostly around 0.0‰. The Δ33S =-0.89‰-＋1.2‰,showing an obvious mass independent fractionation feature. The Δ33S usually displays a negative value at the Algoma-type BIF closely related to volcanic activities,whereas a positive Δ^33S value appears at Superior-type BIF distant from volcanic activity center. The isotopic component of Si and O imply that BIF was formed by submarine exhalation at early stage of the earth,whatever the Algoma-type or the Superior-type. SiO2 was precipitated at first to form the siliceous layer and Fe3O4 subsequently was deposited to form the magnetite layer during the marine exhalation process. A set of rhythmic layer stands for once marine exhalation activity and the periodic marine exhalations form regular rhythmic layers. The widespread BIFs and S isotope mass independent fractionation feature indicate a very low oxygen concentration in the atmosphere back then which may be less than 10^-3 present atmospheric level,intense volcanic and marine exhalation activities,high temperature of sea water,a very high concentration of SiO2 and Fe^2＋,low pH value as well as a 3-5.5,and extremely low soluble sulfate concentration which may be less than 10^-3 present soluble ocean sulfate. After 1.8Ga,the ocean sulfate concentration was increased,and the ocean was from iron rich to sulfate rich that is a decisive factor for the disappearance of the BIFs. The large scale volcanic activities and subma

关 键 词：条带状硅铁建造 硫硅氧同位素 海底喷气 缺氧大气 富铁海洋 

分 类 号：P594[天文地球—地球化学]