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首页> 外文期刊>Contributions to Mineralogy and Petrology >Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton
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Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

机译：根据德兰士瓦州克拉通带状铁形成的铁同位素研究推断，地球上的古代地球化学循环

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摘要

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in 56Fe/54Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the 56Fe/54Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ56Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ56Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ56Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ56Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ56Fe values; these seem best explained through production of high 56Fe/54Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ56Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ56Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.
机译：南非德兰士瓦超级集团的太古宙晚期至元古代带状铁形成（BIF）中铁的同位素组成变化几乎涵盖了整个地球范围，从56的–2.5到+ 1.0‰。 Fe / 54 Fe相对于整个地球的比率。对于56 Fe / 54 Fe比率，当前最新的精度为±0.05‰，该范围是分析误差的70倍，表明可以保留显着的Fe同位素变化。古老的岩石。岩石和矿物的铁同位素组成的显着变化似乎仅限于化学沉淀沉积物，而BIFs的测量范围与火成岩的同位素同质性形成鲜明对比，后者的δ56 Fe = 0.00±0.05‰以及大多数现代黄土，气溶胶，河流负荷，海洋沉积物和元古代页岩。在BIF中测量的赤铁矿，磁铁矿，碳酸铁和黄铁矿的Fe同位素组成似乎反映了以下因素的组合：（1）矿物特定的平衡同位素分馏;（2）从中析出流体的同位素组成的变化; （3）细菌对铁代谢过程的影响。对于可能在初始降水或早期成岩过程中处于同位素平衡状态的矿物，虽然测量结果表明，δ56 Fe值的相对顺序似乎以磁铁矿>菱铁矿>闪锌矿的顺序降低，与光谱数据估计的相似。同位素差异远小于低温下预测的差异。结合正在进行的平衡铁同位素分馏因子的实验确定，BIF矿物的数据对系统Fe（III）-Fe（II）-赤铁矿-磁铁矿-Fe碳酸盐体系的平衡Fe同位素分馏因子施加了更多限制。黄铁矿的δ56 Fe值是天然矿物中测得的最低值，与根据光谱数据预测的高δ56 Fe值形成鲜明对比。一些样品含有赤铁矿和磁铁矿，并且具有正的δ56 Fe值。这些似乎最好通过光合作用的铁氧化产生高56 Fe / 54 Fe储层来解释。目前尚不清楚某些氧化物以及碳酸盐铁的低δ56 Fe值是否反映了生物过程，或低富铁的低δ56 Fe流体中的无机沉淀。但是，目前的结果表明，Fe同位素在追踪Fe的地球化学循环方面具有巨大的潜力，并强调了为确定与沉积环境相关的矿物和流体的平衡Fe同位素分馏因子而需要进行广泛实验的程序。

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  • 来源
    《Contributions to Mineralogy and Petrology》 |2003年第5期|523-547|共25页
  • 作者

    Clark M. Johnson; Brian L. Beard; Nicolas J. Beukes; Cornelis Klein; Julie M. OLeary;

  • 作者单位

    Department of Geology and Geophysics University of Wisconsin;

    Department of Geology and Geophysics University of Wisconsin;

    Department of Geology Rand Afrikaans University Aucklandpark 2006;

    Department of Earth and Planetary Sciences 200 Yale Blvd NE University of New Mexico Albuquerque;

    Department of Geological and Planetary Sciences MC 170-25 1200 E. California Blvd. California Institute of Technology;

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