A re-assessment of the oxidation state of iron in MORB glasses
A re-assessment of the oxidation state of iron in MORB glasses
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DOI：
10.1016/j.epsl.2017.11.032
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发表时间：
2018-02-01
影响因子：
5.3
通讯作者：
Mosselmans, J. Fred W.
中科院分区：
文献类型：
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作者：
Berry, Andrew J.;Stewart, Glen A.;Mosselmans, J. Fred W.
用XANES光谱法测定了大洋中脊玄武岩（MORB）玻璃样品中Fe的氧化态，Fe ~（3+）/Sigma Fe（其中Sigma Fe = Fe ~（2+）+Fe ~（3+））= 0.10（2）（n = 42）。该值低于XANES先前报道的值0.16（1）（n = 103），但与氧化还原滴定法测定的最新值0.11（2）（n = 104）一致，所有这些都是针对相似的样品组。我们将0.16的异常高的XANES值归因于由穆斯堡尔谱的解释和标准品的所得Fe 3 +/Sigma Fe值引起的校准误差。我们的替代解释消除了在玄武岩玻璃中解决Fe 3 +/Sigma Fe值<0.1的问题，产生了与Fe 3 +/Sigma Fe无关的Fe 3+的同质异能素位移和四极分裂值（与Fe 2+的情况一样），并给出了与化学预期的对氧逸度（fO（2））的依赖性一致的Fe 3 +/Sigma Fe值。对于我们的合成MORB组成，Fe 3 +/Fe 2+与fO（2）相关，其与温度无关的表达式为Δ AQFM = 4 log（Fe 3 +/Fe 2+）+4.23（5），其中Δ AQFM是相对于石英-铁橄榄石-磁铁矿缓冲层的以log单位表示的fO-2。天然MORB的平均fO（2）估计为QFM+0.1。(C)2017由Elsevier B. V.出版
The oxidation state of Fe, Fe3+/Sigma Fe (where Sigma Fe = Fe2+ + Fe3+), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) (n = 42). This value is lower than that reported previously by XANES, 0.16(1) (n = 103), but consistent with the most recent value determined by redox titrations, 0.11(2) (n = 104), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of MOssbauer spectra and the resulting Fe3+/Sigma Fe values of the standards. Our alternative interpretation removes the problem of resolving Fe3+/Sigma Fe values < similar to 0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/Sigma Fe (as is the case for Fe2+), and gives Fe3+/Sigma Fe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO(2)). Fe3+/Fe2+ is related to fO(2) for our synthetic MORB composition by the temperature independent expression Delta AQFM = 4 log(Fe3+/Fe2+)+ 4.23(5), where Delta AQFM is the f O-2 in log units relative to the quartz-fayalite-magnetite buffer. The average fO(2) of natural MORB was estimated to be QFM+0.1. (C) 2017 Published by Elsevier B.V.