【作者】 闫斌；

【导师】 朱祥坤；

【作者基本信息】 中国地质科学院 ， 地球化学， 2009， 硕士

【摘要】 陡山陀组地层是新元古代雪球事件以后沉积的第一套地层,记录了这个重要转折期的环境与生物演化信息。我国南方扬子地台陡山沱早期是以碳酸盐岩以及上部的黑色页岩为主的沉积层序。本文选取了不同沉积相的五个剖面进行Fe同位素研究,分别是台地相小峰河、九龙湾和中岭剖面,斜坡相的四都坪剖面,以及深海盆地相的怀化袁家剖面。通过对碳酸盐岩和黑色页岩的铁同位素以及元素地球化学的研究,主要取得以下的成果和认识：1、九龙湾地区处于浅水体系,沉积环境为氧化状态,随着冰川的溶解,海平面逐渐上升,沉积环境转化为弱氧化,反映了雪球事件结束后古海洋水体环境的转变。而四都坪地区一直处于相对深水的体系,沉积环境为还原环境。2、首次在陡山沱组获得碳酸盐岩和黑色页岩的Fe同位素数据。黑色页岩Fe同位素变化范围为δ56Fe=-0.26-0.69‰,平均值为0.24‰。碳酸盐岩Fe同位素变化范围为δ56Fe=-0.67～0.07‰,平均值为-0.34%o。相对于标准物质IRMM-014,所分析的黑色页岩样品显示重同位素富集(除了怀化袁家的黑色页岩),而碳酸盐岩样品富集轻同位素。3、碳酸盐岩样品的Fe同位素在不同沉积相间存在着差异,即使是相同沉积相中,也存在着差别。这说明海水中的铁同位素在空间上是不均一的,受到铁的来源以及沉积环境的影响。对四都坪剖面的盖帽白云岩和黑色页岩夹层的灰岩的Fe同位素研究表明,在所研究的时间段内,海洋的铁同位素组成没有发生明显演化。4、与盖帽碳酸盐相比,九龙湾、小峰河、中岭、四都坪等剖面的黑色页岩都富集Fe的重同位素,表明黑色页岩来自于海水的铁主要是以Fe3+的氧化物或氢氧化物的形式沉淀的。结合微量元素所反映的信息：九龙湾剖面处于弱氧化沉积环境,四都坪剖面处于弱还原环境。怀化袁家剖面的黑色页岩的Fe同位素与盖帽碳酸盐一致,暗示该剖面中来自海水中的铁以亚铁硫化物形式沉淀,说明该剖面形成于强还原环境的静海相(Euxinic)。这表明海洋是分层的,而且陡山沱早期海洋的氧化还原界面应位于九龙湾所处的台地和四都坪所处的大陆斜坡的深度之间。由此推断在陡山沱早期,从台地相、斜坡相到深水盆地相,海水逐渐由表层氧化向深层的还原状态转化。更多还原

【Abstract】 The Doushantuo Formation is the first sedimentary strata after the Neoproterozoic snowball earth, recording the information of the environmental and biological evolution during this critical transition. In Yangtze platform of southern China, the lower Doushantuo strata are mainly composed of carbonate rocks and black shales. Five sections of different sedimentary facies were selected for this study, which are Xiaofenghe, Jiulongwan, Zhongling Sections of the platform facies, Sidouping Section of the shelf facies, and Huaihua-Yuanjia Section of the deep-sea basin facies.Through combination of Fe isotope investigation to some details and elemental characterization on cap carbonates and black shales from these profiles significant advances have been made:1. Ratios of trace elements and enrichment coefficient have been studied. The sedimentary environment is oxidation state in the Jiulongwan area. Following the melt of glaciers and rising of the sea-level, Ancient ocean became deeper. The sedimentary environment changes to weak oxidation, reflecting a transformation of paleo-ocean condition from oxidation to reduction right after the termination of snowball earth event. The Sidouping area lay in a relatively deep water system. Its sedimentary environment is reductive.2. Fe isotope of Doushantuo Formation carbonate rocks and black shales is for the first time measured in this study. Theδ56Fe values of analyzed black shales varies between -0.03%o and 0.69%o with an average of 0.24%o. A range of -0.67%o<δ56Fe<0.07%o is defined by carbonate samples, with an average of -0.34%o. Relative to the IRMM-014 standard, black shales samples show a tendency towards enrichment in heavy isotope (except for the black shale of Huaihua-Yuanjia), whereas carbonate samples are enriched in light isotope.3. Carbonate samples from different sedimentary facies display distinct differences in composition of Fe isotopes even at a single sidementary facies, indicating the iron isotope of seawater is influenced by iron source and sedimentary environment shows a spatial heterogeneity. Fe isotopes of dolomite and limestone samples from Sidouping were analyzed and consistent Fe isotope values were found in these samples. Take Sidouping profile into accout, iron isotope in ocean shows temporal homogeneity.4. The black shales samples from Doushantuo formation (except Huaihua-Yuanjia section) are enriched in heavy isotope. This demonstrates that iron from seawater is mainly controlled by the Fe3+ minerals. Combined with the information reflected by trace elements that Jiulongwan profile is in weak oxide environment. It’s suggested the ocean is stratification. The redox interface should be located between the platform and the slope. The 8’6Fe value of black shales from Huaihua-yujia profile of the deep-sea basin facies is mainly controlled by the sulfide minerals. The environment in the deep-sea basin is in Euxinic. Stratification of ocean is strengthened from platform facies, slope facies to deep-sea basin facies gradually, which transformed from oxide water at surface to Eusintic water in deep-sea gradually.更多还原