【作者】 李志红；

【导师】 朱祥坤；

【作者基本信息】 中国地质科学院 ， 地球化学， 2007， 博士

【摘要】 近年来,随着MC-ICP-MS测试技术的诞生,非传统稳定同位素研究成为快速发展的领域。由于Fe特殊的地球化学、天体化学和生物化学性质,使得铁同位素研究成为重要的国际前沿。但在国内,铁同位素的研究尚属空白。另一方面,前寒武纪铁条带状铁建造不仅是全球最重要的铁矿资源,同时还记录了丰富的古环境信息。为此,本论文在认真总结前人工作的基础上,主要开展了两个方面的研究:建立了MC-ICP-MS铁同位素高精度测定方法,填补了国内空白;对新太古代鞍山—本溪等地区条带状含铁建造进行了较为系统的铁同位素研究,开辟了我国铁同位素地球化学研究新方向。通过上述两个方面的工作,主要取得了以下创新性成果和认识:1.在国内首次建立了Fe同位素高精度测试技术,所建方法的精度达到国际先进水平,从而为我国地学工作者开展Fe同位素地球化学这一前沿研究奠定了坚实基础。2.运用现代测试技术,对鞍山—本溪地区的BIF进行了主量元素和微量元素特征化研究,结果表明,鞍本地区BIF中碎屑组分的含量很低;稀土元素的页岩标准化模式为轻稀土亏损、重稀土富集型,并具有显著的Eu、Y、La正异常说明该区的BIF是古海洋的化学沉积岩,同时具有明显的火山热液的贡献。3.运用所建立的测试方法,对鞍山—本溪地区的BIF进行了比较系统的铁同位素特征化,结果表明:鞍本地区含铁建造的全岩和单矿物均显示较大的Fe同位素变化范围和重同位素富集;相对于磁铁矿,同一样品的黄铁矿显示系统的重Fe同位素富集;与贫矿相比,富铁矿的Fe同位素成分变化范围较小,Fe同位素平均成分较轻。4.讨论了变质作用过程中Fe同位素的地球化学行为,得出了两点基本认识:即区域变质作用使铁同位素在同一样品的磁铁矿和黄铁矿两相及同一矿物相的不颗粒之间发生了再分配,并基本达到了平衡;但BIF的全岩样品在很大程度上保留了原始沉积的Fe同位素信息。5.探讨了氧化还原作用和沉淀程度对Fe同位素分馏的影响:BIF的Fe同位素成分与Fe³⁺/Fe²⁺比值具有负相关的趋势,表明氧化还原是导致Fe同位素分馏的主导因素;模拟计算的结果表明随着海水中铁的沉淀比例的增加,BIF的Fe同位素成分减少;且Fe同位素成分与TFe₂O₃含量存在负相关关系,表明沉淀程度会影响Fe同位素的分馏。6.对新太古代的古环境提出了制约:模拟计算的结果表明海水中只有少部分的铁发生了氧化和沉淀形成BIF,说明在太古代时期,氧化剂的含量是有限的,即当时海洋和大气处于一种缺氧的状态。7.对富矿的成因提出了进一步制约:Fe同位素的成分及Fe同位素成分与TFe₂O₃含量存在负相关关系表明原岩在沉淀过程中发生了比贫矿更高的铁的富集,变质热液活动中因CO₂、SiO₂等组分的带出而发生了铁的进一步富集。8.首次从成矿元素本身为BIF的物质来源提供直接证据:BIF的Fe同位素成分与Eu异常存在非常明显地正相关关系,这清楚地表明鞍本地区BIF中铁的来源与海底火山热液活动有关。9.提出了地球早期海洋中Fe的循环模式:洋中脊喷发富集Fe轻同位素的二价铁热液流体,流体进入海洋后经过部分氧化和沉淀的过程使得BIF富集Fe的重同位素,后期的变质作用使得不同矿物间发生了Fe的均一化及再平衡。更多还原

【Abstract】 Studies of non-traditional stable isotopes have being developed quickly in recent years thanks to the introduction of multi-collector plasma-source mass-spectrometry （MC-ICP-MS）. Researches of iron isotopes have attracted the most noticeable among the non-traditional stable isotopes, due to the special properties of iron in geochemistry, cosmochemistry and biogeochemistry, yet the study of iron isotopes has not been done in China. The banded iron formations （BIF） are not only the most important source of iron ores, but also important archives for the early evolution of atmosphere and hydrosphere of the Earth. This thesis is consisted of two-parts: the development of techniques for high-precision measurement of iron isotope using MC-ICPMS, and application of the techniques developed to the study of iron isotope geochemistry of BIF from Anshan-Benxi area, NE China.The main achievements of the thesis are as following:1. Techniques for high-precision iron measurement have been established, with a precision among the best in the international community.2. The BIFs in Anshan-Benxi have been characterised to some detail in terms of major and trace elements, showing very low contents of TiO₂ and Al₂O₃, and prominent anomalies of Eu , Y, La. This shows that the BIFs are the products of chemical sedimentation from paleo-seawater with significant input of volcanic hydrothermal fluids.3. Detailed characterization of Fe isotopes of the BIFs have been performed. The results show that both bulk samples and mineral separates of magnetite, pyrite and hematite are rich in the heavy Fe isotopes relative to IRMM-014 or the bulk silicate Earth. Furthermore, pyrites in the BIFs show systematic enrichment of heavy isotopes relative to magnetites in the same samples, and the Fe isotope compositions of rich iron ores are lighter and with smaller range of variation thant those of poor iron ore samples4. The behaviors of iron isotopes during metamorphism have been discussed. Whilst re-equilibrium between grains of magnetite and pyrite has been largely achieved at hand-specimen scale, the bulk sample have retained the primary features of Fe isotopes from sedimentation.5. Both redox and precipitation are major factors in controlling iron isotope fractionation, and the range of variations in Fe isotope compositions observed may be explained in terms of different degree of precipitation, which in turn was controlled by the redox state of seawater.6. On the bases of Rayleigh distillation modeling, only small portion of the Fe in seawater has been precipitated, implying that the oxidant was very limited at that time.7. Process for the formation of the rich iron ores has been constrained based on the Fe isotope features observed: iron was more enriched in the precursors of rich-iron ores relative to those of poor-iron ores during sedimentation, and further enrichment was taking place during subsequent metamorphic hydrothermal activities, resulting in the formation of iron-rich ore bodies.8. The source of Fe was provided strong supporting evidence. Iron isotopic ration of BIF has the positive dependence ofδEu, indicating the source of Fe correlation with hydrothermal fluid.9. A model for Fe cycling. The iron isotopic composition lightly from hydrothermal fluid upwelled into ocean, BIFs are enriched in heavy isotopes of Fe by partial oxidation and precipitation, and iron isotopic composition was homogenous and reequilibrium during different minerals by metamorphism.更多还原