Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits
Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits
复制标题
DOI：
10.1016/j.gca.2013.11.018
复制
发表时间：
2014-02
影响因子：
5
通讯作者：
Kirsten Moeller;R. Schoenberg;T. Grenne;I. Thorseth;K. Drost;R. Pedersen
中科院分区：
文献类型：
--
作者：
Kirsten Moeller;R. Schoenberg;T. Grenne;I. Thorseth;K. Drost;R. Pedersen
古硅质铁建造的形成途径及相关的铁同位素分馏作用至今仍不完全清楚。然而，调查这些过程是困难的，因为很好的现代类似物古代铁的形成是稀缺的。现代的硅质羟基氧化铁矿床发现于海底热液喷口，它们是从海底沿着断层和裂缝的扩散低温流体中沉淀出来的。这些矿床表现出的纹理和化学特征，是类似于一些古生代铁地层，提出了一个问题，后者是否可以沉淀从扩散热液流体，而不是从热液plumes.In这项研究中，我们提出的第一个数据，现代铁羟基氧化物矿床从一月马延热液喷口领域，挪威格陵兰海。我们研究的样品显示出非常低的δ 56 Fe值，介于−2.09‰和−0.66‰之间。由于不同程度的部分氧化，铁氢氧化物除了一个例外，要么与它们沉淀的低温热液流体无法区分（δ 56 Fe为-1.84 ‰和-1.53 ‰），要么富含重铁同位素。此外，我们研究了铁同位素变化的奥陶纪碧玉床从Løkken蛇绿岩复杂，挪威，已被解释为代表的成岩产物的硅质水铁矿前体沉淀在热液羽，以比较不同的形成途径的Fe羟基氧化物矿床。相对于现代高温热液喷口流体（约100 ℃），碧玉样品中的铁同位素具有更高的δ 56 Fe值（−0.38‰至+0.89‰）。平均为-0.40 ‰），支持沉降模型。然而，形成的奥陶系碧玉的扩散通风不能排除，由于岩性的差异，地下的两个调查通风system.Our研究表明，可靠的解释铁同位素变化在现代和古代海洋氢氧化铁矿床依赖于全面的知识的地质背景。此外，我们还证明了这些样品中非常负的δ 56 Fe值可能不是微生物异化铁还原的结果，而是由无机反应引起的。
Formation pathways of ancient siliceous iron formations and related Fe isotopic fractionation are still not completely understood. Investigating these processes, however, is difficult as good modern analogues to ancient iron formations are scarce. Modern siliceous Fe oxyhydroxide deposits are found at marine hydrothermal vent sites, where they precipitate from diffuse, low temperature fluids along faults and fissures on the seafloor. These deposits exhibit textural and chemical features that are similar to some Phanerozoic iron formations, raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes.In this study, we present the first data on modern Fe oxyhydroxide deposits from the Jan Mayen hydrothermal vent fields, Norwegian-Greenland Sea. The samples we investigated exhibited very low δ56Fe values between −2.09‰ and −0.66‰. Due to various degrees of partial oxidation, the Fe oxyhydroxides are with one exception either indistinguishable from low-temperature hydrothermal fluids from which they precipitated (−1.84‰ and −1.53‰ in δ56Fe) or are enriched in the heavy Fe isotopes. In addition, we investigated Fe isotope variations in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume, in order to compare different formation pathways of Fe oxyhydroxide deposits. Iron isotopes in the jasper samples have higher δ56Fe values (−0.38‰ to +0.89‰) relative to modern, high-temperature hydrothermal vent fluids (ca. −0.40‰ on average), supporting the fallout model. However, formation of the Ordovician jaspers by diffuse venting cannot be excluded, due to lithological differences of the subsurface of the two investigated vent systems.Our study shows that reliable interpretation of Fe isotope variations in modern and ancient marine Fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context. Furthermore, we demonstrate that very negative δ56Fe values in such samples might not be the result of microbial dissimilatory iron reduction, but could be caused instead by inorganic reactions.