Oxidized iron in garnets from the mantle transition zone
Oxidized iron in garnets from the mantle transition zone
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DOI：
10.1038/s41561-017-0055-7
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发表时间：
2018-01
影响因子：
18.3
通讯作者：
E. Kiseeva;D. Vasiukov;B. Wood;C. McCammon;T. Stachel;M. Bykov;E. Bykova;A. Chumakov;V. Cerantola;J. Harris;L. Dubrovinsky
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文献类型：
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作者：
E. Kiseeva;D. Vasiukov;B. Wood;C. McCammon;T. Stachel;M. Bykov;E. Bykova;A. Chumakov;V. Cerantola;J. Harris;L. Dubrovinsky
地幔中铁的氧化态在大约 200 公里的深度是众所周知的，但尚未在来自上地幔最下部（200-410 公里深度）或过渡带（410-660 公里深度）的样品中进行表征。来自深部（> 200 公里）地幔的天然样品极为罕见，通常仅作为钻石内含物被发现。在这里，我们使用同步加速器穆斯堡尔源光谱辅以单晶 X 射线衍射来测量金刚石中超高压多数石榴石包裹体中 Fe 的氧化态。石榴石的氧化态随着深度的增加而显着增加，Fe 3+/(Fe 3++ Fe 2+) 从约 240 km 深度处的 0.08 增加到约 500 km 深度处的 0.30。后者主要来自辉石岩块状成分，其 Fe 3+ 含量是浅地幔氧化程度最高的石榴石的两倍。相应的氧逸度高于铁金属的稳定上限。这意味着氧化态的增加与 Fe 2+ 歧化为 Fe 3+ 加 Fe 0 无关。相反，Fe 3+ 随深度的增加与碳酸化流体或熔体是导致包裹体中高 Fe 3+ 含量的氧化剂的假设一致。
The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200 km, but has not been characterized in samples from the lowermost upper mantle (200–410 km depth) or the transition zone (410–660 km depth). Natural samples from the deep (> 200 km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe 3+/(Fe 3++ Fe 2+) increasing from 0.08 at approximately 240 km depth to 0.30 at approximately 500 km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe 3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe 2+ to Fe 3+ plus Fe 0. Instead, the Fe 3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe 3+ contents of the inclusions.