A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe
A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe
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DOI：
10.2138/am-2018-6268
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发表时间：
2018-03
影响因子：
3.1
通讯作者：
E. Cottrell;A. Lanzirotti;B. Mysen;S. Birner;Katherine A. Kelley;R. Botcharnikov;F. Davis;M. Newville
中科院分区：
文献类型：
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作者：
E. Cottrell;A. Lanzirotti;B. Mysen;S. Birner;Katherine A. Kelley;R. Botcharnikov;F. Davis;M. Newville
摘要氧逸度（fo 2）对行星内部的地球化学演化具有一级控制作用，硅酸盐玻璃中的Fe ~（3+）/Fe ~（3+）比值为fO 2提供了一个有用的代用指标。Fe K边微X射线吸收近边结构（XANES）光谱使研究人员能够以高精度微观分析确定硅酸盐玻璃的Fe 3 +/Fe 3 O 4 Fe比。在这项研究中，我们表征含水和无水玄武岩玻璃标准穆斯堡尔谱和XANES光谱，并表明同步辐射导致逐步变化的XANES光谱的含水玻璃作为辐射剂量的函数（在这里定义为每平方微米提供的总光子），水的浓度，和初始的Fe 3 +/Fe 3Fe比。我们报告的实验从八个不同的辐射剂量条件下，并表明，在含水硅酸盐玻璃中的铁暴露于辐射后，可以进行快速氧化。氧化速率和氧化程度与辐射剂量、水浓度和亚铁/三价铁氧化物摩尔比的乘积（Φ = XHO 0.5·XFeO/XFeO 1.5）有关。例如，当在约2 × 109光子/s/μm2的标称通量密度下进行几分钟分析时，具有4.9 wt%溶解H2O且穆斯堡尔谱中Fe 3 +/Fe 3 O 4 = 0.19的玄武岩玻璃可能显示Fe 3 +/Fe 3 O 4 ≥ 0.35。这种辐射引起的Fe 3 +/Fe 2 O3比值的增加将导致fO 2的高估约两个数量级，对地质过程的解释产生巨大影响。暴露于辐射的样品区域显示出可测量的氢损失，与辐射诱导的O-H键断裂、相关的H迁移和损失以及Fe 2+氧化一致。该机制与无水玻璃在任何光束条件下均未显示损坏的观察结果一致。低温冷却不会减轻，而是加速铁的氧化。光束损伤的影响似乎会无限期地持续下去。我们在测试的最低光子通量密度（3 × 106光子/s/ μm2）下检测到光束损伤;然而，在通量密度≤6 × 107光子/s/μm2时，由质心（来自XANES光谱）和Fe 3 +/SFe比值（来自穆斯堡尔光谱）定义的含水玻璃校准曲线在穆斯堡尔光谱可达到的准确度内与无水校准曲线无法区分。因此，在低光子通量密度下测量的含水玻璃中公布的Fe 3 +/Fe 3 O 4-Fe比可能在测量不确定度内是准确的，相对于穆斯堡尔谱测量的结果。这些新的结果表明，为了从含水镁铁质硅酸盐玻璃中获得准确的Fe 3 +/Fe 3 O 4 Fe比，首先需要仔细监测XANES光谱随入射剂量的变化（例如，固定能量扫描）。使光束散焦和衰减可以防止铁在镁铁质含水玻璃中的显著氧化。
Abstract Oxygen fugacity (fo2) exerts first-order control on the geochemical evolution of planetary interiors, and the Fe3+/ΣFe ratios of silicate glasses provide a useful proxy for fO2. Fe K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy allows researchers to micro-analytically determine the Fe3+/ΣFe ratios of silicate glasses with high precision. In this study we characterize hydrous and anhydrous basalt glass standards with Mössbauer and XANES spectroscopy and show that synchrotron radiation causes progressive changes to the XANES spectra of hydrous glasses as a function of radiation dose (here defined as total photons delivered per square micrometer), water concentration, and initial Fe3+/ΣFe ratio. We report experiments from eight different radiation dose conditions and show that Fe in hydrous silicate glasses can undergo rapid oxidation upon exposure to radiation. The rate and degree of oxidation correlates with radiation dose and the product of water concentration and ferrous/ferric iron oxide ratio on a molar basis (Φ = XHO0.5·XFeO/XFeO1.5). For example, a basalt glass with 4.9 wt% dissolved H2O and Fe3+/ΣFe = 0.19 from its Mössbauer spectrum may appear to have Fe3+/ΣFe ≥ 0.35 when analyzed over several minutes at a nominal flux density of ~2 × 109 photons/s/μm2. This radiation-induced increase in Fe3+/ΣFe ratio would lead to overestimation of fO2 by about two orders of magnitude, with dramatic consequences for the interpretation of geological processes. The sample area exposed to radiation shows measureable hydrogen loss, consistent with radiation-induced breaking of O–H bonds, associated H migration and loss, and oxidation of Fe2+. This mechanism is consistent with the observation that anhydrous glasses show no damage under any beam conditions. Cryogenic cooling does not mitigate, but rather accelerates, iron oxidation. The effects of beam damage appear to persist indefinitely. We detect beam damage at the lowest photon flux densities tested (3 × 106 photons/s/ μm2); however, at flux densities ≤6 × 107 photons/s/µm2, the hydrous glass calibration curve defined by the centroid (derived from XANES spectra) and Fe3+/SFe ratios (derived from Mössbauer spectra) is indistinguishable from the anhydrous calibration curve within the accuracy achievable with Mössbauer spectroscopy. Thus, published Fe3+/ΣFe ratios from hydrous glasses measured at low photon flux densities are likely to be accurate within measurement uncertainty with respect to what would have been measured by Mössbauer spectroscopy. These new results demonstrate that to obtain accurate Fe3+/ΣFe ratios from hydrous, mafic, silicate glasses, it is first necessary to carefully monitor changes in the XANES spectra as a function of incident dose (e.g., fixed-energy scan). Defocusing and attenuating the beam may prevent significant oxidation of Fe in mafic water-bearing glasses.