Quantifying and correcting the effects of anisotropy in XANES measurements of chromium valence in olivine: Implications for a new olivine oxybarometer
Quantifying and correcting the effects of anisotropy in XANES measurements of chromium valence in olivine: Implications for a new olivine oxybarometer
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DOI：
10.2138/am-2017-5911
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发表时间：
2017-06
影响因子：
3.1
通讯作者：
A. Bell;C. Shearer;P. Burger;M. Ren;M. Newville;A. Lanzirotti
中科院分区：
文献类型：
--
作者：
A. Bell;C. Shearer;P. Burger;M. Ren;M. Newville;A. Lanzirotti
摘要 火成橄榄石中的铬价态比可能包含有关它们结晶的熔体的丰富氧化还原信息。实验表明，橄榄石的 Cr2+/ΣCr 随 fo2 系统变化，因此橄榄石中 Cr 价的测量可用作定量氧压计。对薄片中单个橄榄石斑晶的 Cr 价比进行原位同步加速器 μ-XANES 分析有可能在精细的空间尺度上解锁这种存储的岩浆氧化还原信息。然而，获得结晶材料中阳离子价态的精确 XANES 测量仍然存在障碍，因为这些测量结果可能会受到与样品晶体取向相关的各向异性吸收效应的影响。通过校准各向异性校正来提高橄榄石中 Cr 价比的 XANES 测量精度，是将橄榄石中 Cr 价测量开发为严格的氧气压计的重要一步。为了实现这一目标，我们使用了一种综合方法，将橄榄石中的实验、电子背散射衍射分析和 XANES 测量相结合，系统地研究取向如何影响所得的 Cr K 边缘 XANES 光谱以及从中计算出的 Cr 价比。本工作中生成的数据集用于构建模型，以减轻计算的 Cr2+/ΣCr 值的各向异性影响。将此校正程序应用为光谱处理的一部分，将所得 Cr2+/ΣCr 值的整体精度提高了近五倍。各向异性校正提高了 XANES 测量的 Cr 价比的精度，将计算的 fo2 值的误差从大约 ±1.2 减少到 ±0.25 个对数单位。
Abstract Chromium valence ratios in igneous olivine may hold a wealth of redox information about the melts from which they crystallized. It has been experimentally shown that the Cr2+/ΣCr of olivine varies systematically with fo2 therefore measurements of Cr valence in olivine could be employed as a quantitative oxybarometer. In situ synchrotron μ-XANES analyses of Cr valence ratios of individual olivine phenocrysts in thin section have the potential to unlock this stored magmatic redox information on a fine spatial scale. However, there are still obstacles to obtaining accurate XANES measurements of cation valence in crystalline materials, as the results from these measurements can be compromised by anisotropic absorption effects related to the crystallographic orientation of the sample. Improving the accuracy of XANES measurements of Cr valence ratios in olivine by calibrating an anisotropy correction is a vital step in developing Cr valence measurements in olivine as a rigorous oxybarometer. To accomplish this goal, we have used an integrated approach that combined experiments, electron backscatter diffraction analysis, and XANES measurements in olivine to systematically examine how orientation affects the resultant Cr K-edge XANES spectra and the Cr valence ratios that are calculated from them. The data set generated in this work was used to construct a model that mitigates the effects of anisotropy of the calculated Cr2+/ΣCr values. The application of this correction procedure as a part of spectral processing improves the overall accuracy of the resultant Cr2+/ΣCr values by nearly a factor of five. The increased accuracy of the XANES measured Cr valence ratios afforded by the anisotropy correction reduces the error on calculated fo2 values from approximately ±1.2 to ±0.25 log units.