Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater
Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater
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DOI：
10.1016/j.gca.2010.02.014
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发表时间：
2010-05
影响因子：
5
通讯作者：
M. Nishizawa;Hiroki Yamamoto;Y. Ueno;S. Tsuruoka;T. Shibuya;Y. Sawaki;S. Yamamoto;Y. Kon;
中科院分区：
文献类型：
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作者：
M. Nishizawa;Hiroki Yamamoto;Y. Ueno;S. Tsuruoka;T. Shibuya;Y. Sawaki;S. Yamamoto;Y. Kon;
前寒武纪浅海的氧化还原状态与物质循环和基于光合作用的生态系统的演化有关。铁是一种氧化还原敏感元素，以可溶性 Fe(II) 或不溶性 Fe(III) 形式存在于地球表面。先前的研究表明，海洋沉积矿物的铁同位素比对于了解海洋氧化还原状态很有用，尽管太古代浅海的氧化还原状态知之甚少。这部分是因为经常使用传统的大量同位素分析技术，其中铁同位素记录可能会因同一样品中存在同位素不同的含铁矿物而受到抑制。在这里，我们使用新开发的近红外飞秒激光烧蚀多接收器 ICP-MS 技术 (NIR-fs-LA-MC-ICP-MS) 报告了地质时期浅层海洋叠层碳酸盐中单个黄铁矿颗粒的微尺度铁同位素比率。我们确定同期样品中黄铁矿的晶粒级铁同位素分布呈现双峰（2.7 和 2.3Ga）或单峰模式（2.9、2.6 和 0.7Ga）。特别是，来自 2.7Ga Fortescue 群的黄铁矿显示出独特的双峰分布，具有高正值（+1.0‰ 定义为类型 1）和负 δ56Fe 值（-1.8‰ 定义为类型 2）。 1 型和 2 型黄铁矿有时会出现在同一岩石样本的不同硅质层内。层状铁同位素异质性表明两类黄铁矿的铁同位素比例在沉积后并未因成岩作用而均一化。一些立方黄铁矿的核心具有正 δ56Fe 值（1‰），边缘具有地壳 δ56Fe 值（0‰）。观察到的同位素分带表明，δ56Fe 正值是叠层石形成时的主要特征，而次生黄铁矿在成岩作用期间沉淀。类型1的正δ56Fe值以及类型1和类型2之间较大的铁同位素差异（2.8‰）表明2.7-Ga浅海中发生了部分Fe(II)氧化，即海水中富集56Fe的氢氧化铁（类型1）和贫化Fe2+aq（类型2）的黄铁矿化。 2 型黄铁矿可能不是由成岩过程中微生物铁氧化还原循环产生的，因为这种情况需要 δ56Fe 为 0‰ 的黄铁矿丰度高于 -1.8‰。因此，2.7-Ga浅海中Fe(II)氧化程度可以通过Fe2+aq稳态模型来估计。模型计算表明，一半的Fe2+aq流入量在海水中被氧化。这意味着光合作用产生的 O2 将被 Fe2+aq 流入的氧化完全消耗。黄铁矿的颗粒尺度铁同位素分布可能是重建太古代浅海氧化还原态的有用指标。
The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS). We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3Ga) or unimodal pattern (2.9, 2.6, and 0.7Ga). In particular, pyrite from the 2.7Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ56Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ56Fe value (1‰) and a rim with a crustal δ56Fe value (0‰). The observed isotopic zoning suggests that the positive δ56Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis. The positive δ56Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of56Fe-enriched ferric oxyhydroxide (Type 1) and56Fe depleted Fe2+aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ56Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe2+aq steady-state model. The model calculation shows that half the Fe2+aq influx was oxidized in the seawater. This implies that O2produced by photosynthesis would have been completely consumed by oxidation of the Fe2+aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.