条带状铁建造:特征、成因及其对地球环境的制约

条带状铁建造(BIF)是由硅质和铁质组成的化学沉积岩,通常具有典型的薄层或薄板状构造。BIF不仅记录了丰富的岩石圈、大气圈、水圈和生物圈状态及演化的信息,而且与其相关的铁矿也是世界上最重要的铁矿资源类型。虽然对BIF的研究已取得了丰富的成果,近年来若干新颖的观点和重要的发现对进一步深入研究BIF成因提供了重要思考途径和研究线索。文章总结了BIF类型、矿物学和地球化学特征,分析了BIF形成条件及大规模缺失机理,阐述了BIF的形成和大气圈两次重大氧化事件(古元古代大氧化事件,2.4～2.1 Ga,新元古代氧化事件,0.8～0.55 Ga?)之间的关系,认为BIF与海底岩浆-热液活动具有密切的成因联系。最后,提出BIF的沉积和消亡与重大地质事件密切相关,这些重大地质事件也相互作用,共同制约着大气海洋环境演化、地球生物学和地球动力学过程。关于BIF精细成矿时代、条带/条纹形成的机制、重大地质事件对BIF形成的制约和BIF微生物成因等方面仍存在较多问题。随着大数据、精细成矿年代学、模拟实验、非传统稳定同位素(Fe、Cr、U、Mo和Cu同位素等)新技术方法的进步,及对现代洋底热液系统的深入认识,将有助于揭开BIF成因的神秘面纱。

Banded iron formations: Their characteristics, genesis and implications for ancient Earth's environment

Banded Iron Formation (BIF) is iron-rich and siliceous chemical sedimentary rocks, which show typical thin bedded or laminated structure. BIF has recorded state and evolution of lithosphere, atmosphere, hydrosphere and biosphere of the Earth, and, what is more, its related iron ore deposit is also an important iron ore source. Although the study of BIF has made remarkable achievements, both newly novel viewpoints and important discove-ries provide many important thinking approaches and researching clues for the further understanding of the origin of BIF. The characteristics, mineralogical and geochemical features of BIF were summarized in this study. Meanwhile, the formation conditions and the mechanism of disappearance of BIF were discussed. The authors also discussed the relationship between BIF deposition and two-step rise of oxygen in Earth''s atmosphere (Paleoproterozoic Great Oxidation Event, 2.4~2.1 Ga, and Neoproterozoic Oxygenation Event, 0.8~0.55 Ga?), and argued that the formation of BIF was genetically linked to submarine magmatism and hydrothermal activities. Finally, the authors propose that the emerging and disappearance of BIF were intimately associated with major geological events, which complicatedly reacted with each other and influenced the evolution of atmosphere-ocean conditions, geobiology and geodynamics. However, the precise depositional age, formation mechanism of band/lamina and the problem as to how major geological events constrained the formation of BIF and microbiological origin of BIF are also poorly understood. With the application of big data, precise chronology, laboratory experiments, progress in new technologies of nontraditional stable isotopes (e.g., Fe, Cr, U, Mo, Cu isotopes) and further investigations of modern seafloor hydrothermal system, the genesis of BIF will be surely unveiled step by step.