Iron isotopes are a valuable tool for evaluating processes that control Fe redox cycling in modern and ancient environmental settings. However, robust evaluation of Fe isotope compositions in natural samples requires that fractionations associated with key (bio)geochemical reactions are well-defined. The reductive dissolution of Fe (oxyhydr)oxide minerals mediated by dissolved sulfide exerts a major influence on solid phase Fe mineralogy and dissolved porewater Fe profiles during early diagenesis of organic-rich sediments, but to date, no studies have investigated Fe isotope fractionations during this process. Here, we report the results of laboratory sulfidation experiments, examining apparent Fe isotope fractionations for a variety of Fe (oxyhydr)oxide minerals. The iron isotope compositions of reaction products were determined for both the reduction-dominated and dissolution-dominated steps of the reaction. The reductive step for lepidocrocite and hematite produced Fe(II) that was up to 0.25 ‰ heavier than the bulk starting mineral. By contrast, the reduction of ferrihydrite produced isotopically light Fe(II), with isotope compositions −0.1 to −0.6 ‰ lower than the initial mineral. Consistent with previous studies of the reductive dissolution of Fe (oxyhydr)oxide minerals via abiological and biological pathways, the lighter isotope was preferentially released from the mineral surface during the dissolution phase for all minerals, with dissolved Fe isotope compositions up to ∼2.0 ‰ lower than the surface bound Fe(II). The magnitude of isotopic fractionation during both of these steps is directly related to rates of reaction, and is thus controlled by factors such as sulfide concentration, mineral concentration, crystal structure, surface area and pH. Our data demonstrate that dissolved Fe with δFe compositions approaching −1.0 ‰ is readily generated during the overall reaction, suggesting that sulfide-promoted reductive dissolution of Fe (oxyhydr)oxide minerals may contribute significantly to the generation of light Fe isotope compositions in anoxic settings.

中文翻译：

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硫化物促进氧化铁（羟基）氧化物矿物还原溶解过程中的铁同位素分馏

铁同位素是评估现代和古代环境中控制铁氧化还原循环过程的重要工具。然而，对天然样品中铁同位素组成的稳健评估需要明确定义与关键（生物）地球化学反应相关的分馏。由溶解硫化物介导的 Fe（羟基）氧化物矿物的还原溶解对富含有机物沉积物早期成岩作用期间的固相 Fe 矿物学和溶解孔隙水 Fe 剖面产生重大影响，但迄今为止，还没有研究调查在此期间的 Fe 同位素分馏过程。在这里，我们报告了实验室硫化实验的结果，检查了各种铁（羟基）氧化物矿物的表观铁同位素分馏。测定了反应的还原主导步骤和溶解主导步骤的反应产物的铁同位素组成。纤铁矿和赤铁矿的还原步骤产生的 Fe(II) 比块状起始矿物重 0.25 ‰。相比之下，水铁矿的还原产生同位素轻 Fe(II)，其同位素组成比初始矿物低 -0.1 至 -0.6 ‰。与之前通过非生物和生物途径对 Fe（羟基）氧化物矿物还原溶解的研究一致，所有矿物在溶解阶段均优先从矿物表面释放出较轻的同位素，溶解的 Fe 同位素成分低约 2.0 ‰比表面结合的 Fe(II) 。这两个步骤中同位素分馏的程度与反应速率直接相关，因此受硫化物浓度、矿物质浓度、晶体结构、表面积和 pH 等因素控制。我们的数据表明，在整个反应过程中很容易产生δFe成分接近-1.0‰的溶解Fe，这表明硫化物促进的Fe（羟基）氧化物矿物的还原溶解可能对缺氧环境中轻Fe同位素成分的产生有显着贡献。