Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770 Ma Isua Supracrustal Belt (West Greenland)
Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770 Ma Isua Supracrustal Belt (West Greenland)
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DOI：
10.1016/j.epsl.2012.12.025
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发表时间：
2013-02-01
影响因子：
5.3
通讯作者：
Moorbath, Stephen
中科院分区：
文献类型：
--
作者：
Czaja, Andrew D.;Johnson, Clark M.;Moorbath, Stephen
太古代大气-海洋系统的氧化还原平衡是我们对地球表面带最早历史的理解中最重要的不确定性之一。大多数研究者都认为，直到公元前2000年，氧气才在大气中占很大比例。2.45 Ga，在大氧化事件之后，但是关于O-2的产生何时开始以及如何被海洋中的还原物种（如Fe（II））消耗的协议较少。铁的氧化还原循环随着时间的推移已被跟踪使用带状铁地层（BIFs），和铁同位素越来越多地用于约束地球的古环境条件，包括形成BIFs的途径。铁同位素分析的BIF从3.7至3.8 Ga Isua表壳带（ISB），通过富磁铁矿层的微取样和常规分析，以及通过原位飞秒激光烧蚀（fs-LA-ICP-MS），表明一个一致的非零Δ Fe-56值的范围很窄。通过fs-LA-ICP-MS分析磁铁矿允许精确和准确的微米级分析，而没有与二次离子质谱（西姆斯）分析相关的取向效应的问题。磁铁矿δ Fe-56值在不同带之间的范围为+0.4 ‰至+1.1 ‰，但在单个层内磁铁矿颗粒大多是均匀的。尽管这些BIF已变质为角闪岩相，但变质作用既不能解释带间Fe同位素组成的变化，也不能解释手样之间的变化。因此，同位素组成反映了“原始”的低温沉积值。从ISB磁铁矿测得的正δ Fe-56值最好用富Fe（II）海水部分氧化产生的Fe（III）氧化物沉积来解释。一个分散/反应模型解释了水热Fe（II）（aq）输入速率、氧化速率和Fe（OH）（3）沉降速率，表明O-2含量异常低，
The redox balance of the Archean atmosphere-ocean system is among the most significant uncertainties in our understanding of the earliest history of Earth's surface zone. Most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca. 2.45 Ga, after the Great Oxidation Event, but there is less agreement on when O-2 production began, and how this may have been consumed by reduced species such as Fe(II) in the oceans. The Fe redox cycle through time has been traced using banded iron formations (BIFs), and Fe isotopes are increasingly used to constrain the conditions of Earth's paleoenvironments, including the pathways of formation of BIFs. Iron isotope analyses of BIFs from the 3.7 to 3.8 Ga Isua Supracrustal Belt (ISB), obtained by micro-sampling of magnetite-rich layers and conventional analysis, as well as by in situ femtosecond laser ablation (fs-LA-ICP-MS), indicate a consistently narrow range of non-zero delta Fe-56 values. Analysis of magnetite by fs-LA-ICP-MS allows for precise and accurate micron-scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry (SIMS) analyses. Magnetite delta Fe-56 values range from +0.4 parts per thousand to +1.1 parts per thousand among different bands, but within individual layers magnetite grains are mostly homogeneous. Although these BIFs have been metamorphosed to amphibolite-facies, the metamorphism can neither explain the range in Fe isotope compositions across bands, nor that between hand samples. The isotopic compositions therefore reflect "primary", low-temperature sedimentary values. The positive delta Fe-56 values measured from the ISB magnetites are best explained by deposition of Fe(III)-oxides produced by partial oxidation of Fe(II)-rich ocean water. A dispersion/reaction model, which accounts for rates of hydrothermal Fe(II)(aq) input, rates of oxidation, and rates of Fe(OH)(3) settling suggests exceptionally low O-2 contents,