Abstract Fe oxidation/reduction reactions play a fundamental role in a wide variety of geological processes. In natural materials, Fe redox state commonly varies across small spatial scales at reaction interfaces, yet the approaches available for quantitatively mapping the Fe redox state at the microscale are limited. We have designed an optimized synchrotron-based X‑ray spectroscopic approach that allows microscale quantitative mapping of Fe valence state by extending the Fe XANES pre-edge technique. An area of interest is mapped at nine energies between 7109–7118 eV and at 7200 eV, allowing reconstruction, baseline subtraction, and integration of the pre-edge feature to determine Fe(III)/ΣFe with 2 μm spatial resolution. By combining the Fe redox mapping approach with hyperspectral Raman mineralogy mapping, the Fe oxidation state distributions of the major mineral phases are revealed. In this work, the method is applied to a partially serpentinized peridotite with various Fe-bearing secondary mineral phases to trace the Fe transformations and redox changes that occurred during its alteration. Analysis with the Fe redox mapping technique revealed that the peridotite contained relict olivine with abundant Fe(II), while serpentine, pyroaurite, and another hydroxide phase are secondary mineral reservoirs of Fe(III). Although serpentine is not Fe-rich, it contained approximately 74% ± 14% Fe(III)/ΣFe. These analytical results are integral to interpreting the sequence of alteration reactions; serpentinization of primary olivine formed Fe(II)-rich brucite and oxidized serpentine, which could have contributed to H2 production during serpentinization. Subsequent weathering by oxidizing, CO2-bearing fluids led to the partial carbonation and oxidation of brucite, forming pyroaurite and a hydroxide phase containing dominantly Fe(III). This Fe redox imaging approach is applicable to standard petrographic thin sections or grain mounts and can be applied to various geologic and biogeochemical systems.

中文翻译：

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通过多能量 X 射线荧光映射在 Fe K 前边缘峰进行定量微尺度 Fe 氧化还原成像

摘要 Fe 氧化/还原反应在各种地质过程中起着重要作用。在天然材料中，Fe 氧化还原状态通常在反应界面的小空间尺度上变化，但可用于在微观尺度上定量映射 Fe 氧化还原状态的方法是有限的。我们设计了一种优化的基于同步加速器的 X 射线光谱方法，通过扩展 Fe XANES 前边缘技术，允许对 Fe 价态进行微尺度定量映射。感兴趣的区域以 7109-7118 eV 和 7200 eV 之间的九个能量进行映射，允许重建、基线减法和前边缘特征的集成，以确定具有 2 μm 空间分辨率的 Fe(III)/ΣFe。通过将铁氧化还原映射方法与高光谱拉曼矿物映射相结合，揭示了主要矿物相的 Fe 氧化态分布。在这项工作中，该方法应用于具有各种含铁次生矿物相的部分蛇纹石化橄榄岩，以追踪在其蚀变过程中发生的 Fe 转变和氧化还原变化。用铁氧化还原绘图技术分析表明，橄榄岩含有富含 Fe(II) 的残存橄榄石，而蛇纹石、焦金石和另一种氢氧化物相是 Fe(III) 的次生矿物储层。尽管蛇纹石不富含铁，但它含有大约 74% ± 14% 的 Fe(III)/ΣFe。这些分析结果对于解释改变反应的顺序是不可或缺的；初级橄榄石的蛇纹石化形成了富含 Fe(II) 的水镁石和氧化的蛇纹石，这可能有助于蛇纹石化过程中的 H2 产生。随后通过氧化、含 CO2 流体的风化导致水镁石的部分碳酸化和氧化，形成焦金石和主要含有 Fe(III) 的氢氧化物相。这种 Fe 氧化还原成像方法适用于标准的岩相薄片或颗粒镶嵌，并可应用于各种地质和生物地球化学系统。