Iron Isotope Geochemistry Of Banded Iron Formations From Anshan-Benxi Area, NE China

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.