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 Fe³⁺/(Fe³⁺+ Fe²⁺) 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 Fe³⁺ 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 Fe²⁺ to Fe³⁺ plus Fe⁰. Instead, the Fe³⁺ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe³⁺ contents of the inclusions.

铁在地幔中的氧化态众所周知，深度约为200 km，但尚未从最下部的上地幔（深度200–410 km）或过渡带（深度410–660 km）中进行表征。深部（> 200 km）地幔中的天然样品极为罕见，通常仅被发现是钻石中的内含物。在这里，我们使用同步加速器穆斯堡尔谱仪，辅以单晶X射线衍射，测量金刚石中超高压专业石榴石夹杂物中的铁的氧化态。石榴石的氧化态随深度的增加而显着增加，其中Fe ³⁺ /（Fe ³⁺ + Fe ²⁺）从大约240公里深度处的0.08增加到大约500公里深度处的0.30。后者的主要成分来自于辉铁铁矿的整体成分，其Fe ^3+的富集程度是来自浅地幔中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性上限。这意味着氧化态的增加与Fe ²⁺向Fe ³⁺加Fe ^0的歧化无关。相反，Fe ³⁺随深度的增加与碳酸盐流体或熔体是造成夹杂物中高Fe ³⁺含量的氧化剂这一假设相一致。