| 古元古代大氧化事件(GOE)前后海洋环境的变化: 来自华北条带状铁建造(BIF)岩相学和地球化学的证据* |
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| 引用本文： | 张连昌,兰彩云,王长乐,彭自栋,佟小雪,李文君,董志国.古元古代大氧化事件(GOE)前后海洋环境的变化: 来自华北条带状铁建造(BIF)岩相学和地球化学的证据*[J].古地理学报,2020,22(5):827-840. |
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| 作者姓名： | 张连昌 兰彩云 王长乐 彭自栋 佟小雪 李文君 董志国 |
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| 作者单位： | 1.中国科学院地质与地球物理研究所,中国科学院矿产资源研究重点实验室,北京 100029;2.中国科学院大学,北京 100049;3.西北大学地质学系,大陆动力学国家重点实验室,陕西西安 710069 |
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| 基金项目： | *国家自然科学基金项目(编号: U1812402,41972199)资助 |
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| 摘 要： | 条带状铁建造(BIF)是形成于前寒武纪海洋中的化学沉积岩,记录了古海洋氧化还原状态的重要信息。华北克拉通广泛分布的新太古代和古元古代BIF,是了解古元古代大氧化事件(GOE)前后古海洋氧化还原环境变化的理想对象。初步研究表明,华北克拉通新太古代BIF主要为磁铁矿型氧化物相和硅酸盐相,极少数出现碳酸盐相;古元古代BIF包括赤铁矿型和磁铁矿型氧化物相、硅酸盐相和碳酸盐相,其中赤铁矿相是古元古代BIF独有的。以上矿物学特征表明,新太古代和古元古代水体的氧化还原条件是不同的。华北克拉通新太古代BIF的稀土元素组成缺乏强烈的负Ce异常,反映同期海水氧含量非常低,为缺氧状态; 但少量BIF也包含有负Ce异常,同时具有较大变化范围的Th/U值,指示新太古代海洋的局部水体氧含量相对较高,呈弱氧化状态。与新太古代BIF相比,古元古代BIF的Ce异常变化较大,包括无异常、正异常和负异常,尤其是赤铁矿相BIF具明显的负Ce异常,表明古元古代水体的氧含量和氧化还原结构已发生了明显变化; 结合华北克拉通BIF的Ni/Co、V/(V+Ni)和Th/U等比值特征,认为古元古代海洋呈次氧化—氧化环境。新太古代BIF 强烈富集重铁同位素,S同位素非质量分馏效应较为明显;而古元古代BIF相对富集轻铁同位素,S同位素非质量分馏效应不明显。综上,新太古代海洋环境整体缺氧,但局部可能存在氧气“绿洲”,暗示光合产氧作用在太古代晚期已经存在;大氧化事件期间及之后的古海洋总体具上部氧化、下部还原的分层特征。
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| 关 键 词： | 条带状铁建造 前寒武纪 大氧化事件 海洋环境 华北克拉通 |
| 收稿时间： | 2020-07-14 |
Changes of oceanic environment before and after the Paleoproterozoic Great Oxidation Event(GOE): Evidence from petrography and geochemistry of banded iron formation(BIF)from the North China Craton |
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| Zhang Lian-Chan,Lan Cai-Yun,Wang Chang-Le,Peng Zi-Dong,Tong Xiao-Xue,Li Wen-Jun,Dong Zhi-Guo.Changes of oceanic environment before and after the Paleoproterozoic Great Oxidation Event(GOE): Evidence from petrography and geochemistry of banded iron formation(BIF)from the North China Craton[J].Journal of Palaeogeography,2020,22(5):827-840. |
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| Authors: | Zhang Lian-Chan Lan Cai-Yun Wang Chang-Le Peng Zi-Dong Tong Xiao-Xue Li Wen-Jun Dong Zhi-Guo |
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| Institution: | 1.Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;2.University of Chinese Academy of Sciences,Beijing 100049,China;3.Department of Geology,Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China |
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| Abstract: | Banded iron formation(BIF)belongs to sedimentary rocks formed in Precambrian marine,which can directly reflect the redox state of the ancient oceans. Mineral composition and geochemistry of BIF can reveal the relative changes of oxygen contents of ancient atmosphere-ocean. The Neoarchean and Paleoproterozoic BIFs widely distributed in the North China Craton(NCC),are the ideal research objects for understanding the changes of the ancient ocean redox environment before and after the Paleoproterozoic Great Oxidation Event(GOE). Our previous studies indicated that the sedimentary facies of the Neoarchean BIF in the NCC are mainly magnetite-type oxide and silicate,with minor carbonate. The sedimentary facies of the Paleoproterozoic BIF are hematite- and magnetite-type oxide,silicate and carbonate,of which the hematite-oxide facies is unique to the Paleoproterozoic BIF. The above mineralogical features suggest that the redox conditions of the Neoarchean and Paleoproterozoic seawater are different. The rare earth element composition of the Neoarchean BIF in the NCC lacks a strong negative Ce anomaly,reflecting that the oxygen content of contemporary seawater is very low and the marine is anoxic. However,a small amount of BIFs in the NCC also present the negative Ce anomalies and a wide range of Th/U ratios,indicating that the local water of the Neoarchean ocean had relatively high oxygen content and was at a weak oxidation state. Compared with the Neoarchean BIFs,the Paleoproterozoic BIFs present a wide range of Ce anomalies(i.e.,no Ce anomalies,positive Ce anomalies and negative Ce anomalies). The hematite-bearing BIF has an obvious negative Ce anomalies,implying that the oxygen content and redox state of Paleoproterozoic seawater changed significantly. Combined with the ratios of Ni/Co,V/(V+Ni)and Th/U of the BIFs in the NCC,the Paleoproterozoic oceans exhibited a suboxidation to oxidation environment. Besides,Neoarchean BIF is strongly enriched in heavy iron isotopes and the non-mass fractionation of S isotope is obvious,whereas the Paleoproterozoic BIF is relatively enriched in light iron isotopes and the non-mass fractionation of S isotope is not obvious. It is summarized that the Neoarchean marine is anoxic,but the oxygen‘oasis' may exist locally,implying that photosynthetic oxygen production already existed in the Late Neoarchean. The ancient ocean presented a layered characteristics during and after the GOE,indicating that the shallow water was generally oxidized and the deep water was reduced. |
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| Keywords: | banded iron formation Precambrian Great Oxidation Event ocean environment North China Craton |
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