Primary Fe isotope signatures record oxidative precipitation in 3.2 Ga ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean environment

IF 3.2 2区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
Ryohei Suzumeji , Tsubasa Otake , Daizo Yamauchi , Yoko Ohtomo , Takeshi Kakegawa , Christoph Heubeck , Shin-ichi Yamasaki , Tsutomu Sato
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引用次数: 0

Abstract

Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for ca. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe2+-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ56Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ56Fe values (δ56Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe2O3(T)/Al2O3 ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe2O3(T)/Al2O3 – δ56Fe and matrix ratio – δ56Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe2+-silicate precipitation from dissolved ferrous Fe (Fe2+(aq)) was much smaller (Δ56FeFe2+-silicate–Fe2+(aq) < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe2+(aq) supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ13Ccarb = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe3+-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe2O3(T)/Al2O3 – δ56Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe2+ in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment.
原生铁同位素特征记录了浅海环境中沉积的 3.2 Ga 铁屑硅质沉积岩的氧化沉淀过程
铁地层(IFs)的铁(Fe)同位素组成有可能制约早期地球的海洋氧化还原环境和海洋生物圈。然而,对铁地层中铁同位素比值的解释存在争议,这与铁的来源、原生沉淀模式以及随后的矿物转化等多种因素有关。本文介绍了约 3.2 Ga 未风化铁矿石的全岩铁同位素数据。本文介绍了沉积于南非巴伯顿绿岩带穆迪斯群下部(MdI1单元)浅海中的约3.2 Ga未风化铁屑硅质沉积岩的全岩铁同位素数据。我们还通过实验研究了铁2+硅酸盐(如绿泥石)在沉淀过程中的铁同位素效应。铁同位素数据显示,不同岩性(即富磁铁矿粉砂岩、富碳酸盐粉砂岩、砂质粉砂岩和绿泥石)的铁同位素有显著差异(δ56Fe = -0.58 ‰至 +0.60 ‰)。富磁铁矿粉砂岩的δ56Fe值(δ56Fe = -0.54 ‰至 +0.60 ‰)随着Fe2O3(T)/Al2O3比值和基质比值(粒度为<30 μm的碎屑颗粒所占百分比)的减小而减小。富碳酸盐粉砂岩也与富磁铁矿粉砂岩具有相同的 Fe2O3(T)/Al2O3 - δ56Fe 和基质比 - δ56Fe 趋势。合成实验表明,溶解亚铁(Fe2+(aq))在无氧 Fe2+-硅酸盐沉淀过程中的同位素分馏(Δ56FeFe2+-硅酸盐-Fe2+(aq) < +0.3‰)远小于氧化沉淀过程。这些结果表明,富铁粉砂岩(磁铁矿和富碳酸盐粉砂岩)中的铁同位素变化只能用深海提供的 Fe2+(aq)经过雷利型分馏后的氧化沉淀来解释。富碳酸盐粉砂岩的碳酸盐-C同位素比值较低(δ13Ccarb = -5.8 ‰至-3.7 ‰),这表明磁铁矿和角闪石或镁菱铁矿是由含Fe3+的原生矿物在Fe埋藏后通过有机C的氧化作用形成的。磁铁矿和富含碳酸盐的粉砂岩的 Fe2O3(T)/Al2O3 - δ56Fe 变化趋势一致,这表明在含铁矿物的成岩和/或变质转化过程中,铁的还原对整个岩石铁同位素组成的变化微乎其微,这可能是因为 Fe2+ 在沉积物中的流动性有限。因此,铁同位素组成主要记录了发生在 3.2 Ga 浅海环境水柱中的原生沉淀过程。
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来源期刊
Precambrian Research
Precambrian Research 地学-地球科学综合
CiteScore
7.20
自引率
28.90%
发文量
325
审稿时长
12 months
期刊介绍: Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.
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